In terms of RDM, at least post 41~ when convert and refresh are available, who has the best stats or the best mp pool or any of that doesn't really matter. The most important aspect of RDM is managing your mp. The best RDMs accomplish all of their responsibilities (Haste, Refresh, Debuffing, Dispelling and Curing, etc) without exhausting their mp pools entirely between converts. One of the biggest mistakes some bad RDM's I've seen make is not resting in between fights to get that extra tick or two of mp. It doesn't hurt. Someone once told me, the best rdm is one that the party doesn't even notice exists. They perform everything without any strain on their mp. They don't have to be asked for haste or refresh, the party doesn't struggle because the mob is fully debuffed. They cure when necessary. The fights go smoothly with very little downtime. They perform everything that is necessary and expected of them.
Endgame equipment and merits play a much more vital role of course, but the race you choose isn't overly important in that either. Most regular mobs in the game will rarely resist an averagely equipped RDM. NM's and endgame monsters may prove as a bit more of a challenge for the average rdm though, which then merits and gear can easily compensate that.
Now about /blm and /whm. For most situations, I prefer having /whm as a subjob. There are endgame situations where /drk is preferred for chain-stun and extra stuns. /nin is awesome for soloing. /whm and /blm probably have the most widespread effectiveness as a subjob however.
I really enjoy the /blm subjob, primarily because of the Job Traits it grants. I am personally a huge fan of the Conserve MP trait. Having leveled both BLM and RDM to 75, BLM being 75 first, the one thing I miss on RDM is my Conserve MP and my multiple Clear Mind bonuses. For that reason, BLM can be a more viable sub job. There also is Elemental Seal, which basically ensures that a debuff like Sleep will not get resisted. Going from BLM to RDM, I truly miss all of the Clear Mind bonuses that Black Mage has (5 total from 1-75; RDM gets 3, the last one coming at 75), so the Conserve MP trait that /blm gives is pretty decent.
The reason I prefer /whm however is the fact that in many situations, its much easier and cost-efficient to cast curagas or aoe spells in general. The subjob also increases your MND, making for more potent debuffs. Cure IV's or multiple Cure III's can eat up your manapool pretty quickly. Divine Seal Curaga's are useful, but my primary use for Divine Seal typically becomes spike-max healing myself. It usually takes ~2 Cure IV's to cure myself up after a convert. I make it so that I have excess mp on convert, which I use as my Cure IV mp. After switching back to my enfeebling gear, my hp/mp ratio is ~100% full. The issue with DS Cure IV is that it can be a hate spike, so be cautious when doing it. Another biggie...... Please use stoneskin when you convert --- even if theres almost no chance of you dying, its embarassing to die when you convert =P
I'd highly suggest always using /whm pre-40. It's just easier and you don't really get the best benefit of /blm until the sub is 20+. Warp doesn't count as a benefit
Another good thing to know - if you have 0mp and convert, you will waste convert. 700hp/0mp -> Convert -> 700hp/0mp and you're asking yourself wtf happened. Unless they changed that during my break from the game, this is a good thing to know. It's one pro-arguement for rdm's having some form of equippable or edible form of refresh on them at all times. Generally you will have refresh on when you convert, which is desired. But if you aren't watching it well and Refresh goes down and you use that last 40mp to haste the nin or refresh the whm and you're stuck at 0... theres no way to get that 1mp required for a successful convert aside from taking a knee. While 0mp with no refresh isn't common, it does happen at times. It's good to be aware.
A large mp pool does not equate to good mp management. Tarutaru especially with their lower hp, especially when Zenith-gear is tacked on. If a taru has 700hp and 1100mp, they will only maintain that 1100mp at the start of a fight or after a resting period. Otherwise unless they manipulate their gear well, they will maintain ~700hp/mp consistantly AFTER a convert (without curing themselves up either, which will reduce the base amount further). While it isn't the exact situation and there are many tarutaru who can accomplish a nice convert ratio, it is a example of why you shouldn't necessarily go for the top number of mp only when considering RDM. Uzor's(I think) example of an Elvaan with 1000hp/1100mp would be an okay result. But for a taru that has 800~hp with a manapool of 1000mp, thats 200 mp that is pretty much being wasted. My convert ratio is 950hp-920mp (with my mp gear I think, I'm at work atm). When I convert my mp is 920, but after I cure myself a few times I swap back to my enfeebling gear and my MP sits around 780. High MP is good for show but it doesn't compensate if you don't manage it well, especially if you can't maintain that number. I have 900~mp after a convert, but a lot of it goes toward curing myself after a convert. I'm definitley not the perfect RDM though and I do sometimes overshoot my cooldowns. I know some RDMs who feel the need to use convert every 10 minutes or its "being wasted." I prefer to use it "as necessary" and try to maintain enough mp to function via resting and refresh. It all depends on your own playstyle, but that is what works best for me.
In general, the race you choose should depend on what you want it to be, not the stats available. If you want to be a tarutaru and /panic your ass off and hump peoples legs, that works fine. If you want to be a scary elvaan with a wavy Warlock's Tabard, you should do that. If you want to be a neutral hume, do it. Mithra, Galka, do what fits you and makes you happy. I ultimately picked a female hume as my model because I graduated from a MUD style of game where most of it was text based, and my hume model represents the way I always thought my character would look. I didn't really think much of stats at the time. Your character in this game becomes your identity. You might better select a race that you want and not one that has the best stats.
Kind of a more well rounded view of what I think of the rdm job I guess, sorry if its a bit wordy and a little misguided... ;x