Heh, welcome to the club ^_^.
Alright, first, I'm going to start off with the standard BST warning to people new to the job:
Lvl 1-35 is hell. Not just normal hell, imagine you're in Valkurm dunes for 35 levels, in a bad party where the tank can't hold hate, the WHM isn't using ANY MP gear and can't cure without pulling hate. Not to mention you're only making about 1500-2000 exp per hour. That is what BST 1-35 is.
Yes, it sucks. There will be times when you can't take the frustrations of the job. Without leave you will automatically take a 30% exp loss unless you let your pet die first. Charming can be a bit of a pain because there isn't a whole lot of chr+ available until level 15, and even then things can get rough. In fact most people who play BST end up quitting by level 20.
However. Playing BST is a lot of fun. The exp is slow until level 35. There are levels in there where it will be fast. Typically when taking on goblins and bombs. Suicides can net large amounts of exp and high chains, no matter what hte level of BST. But for the most part it will be slow. Once you hit 35 though...
The entire job changes. Once you hit 35 you get leave. You can release your pet at the last sliver of the mobs HP and kill the mob off for full exp. You can also swap between 2 or 3 pets to take down tough enemies (At level 46 I was able to take down an IT++ Golem in Zi'tah by myself). You will for the most part be able to get at least a chain 3 providing you have enough pets and the mobs aren't rediculously tough.
Aggro and linking becomes a thing of the past. Get a link on a pull? As long as whatever linked is charmable, you can leave your current pet, charm the link, and move to a safer area. Crisis averted. Get aggro from that VT Goblin? Keep swapping pets till the sucker suicides or dies off.
Mobs regen HP at a very accellerated rate, something like 5-6 times faster than we can, so you never need to kill off your pets.
Okay, so those are basically the perks of the job. Complete soloability, link control, and post 35 you can easily make 3-4k exp an hour (depending on the mobs you fight).
If you intend to take BST up, don't focus on chains or large exp. That gets you killed. I mean you could, but if you want relatively fast, safe, exp, stick to what is safe. Some levels you have no choice but to stay in an area and walk around with an EM pet and slaughter countless DC-EP mobs. Focus on the little things, some noteable goals in a BST career:
lvl10- you get gauge and heel.
lvl 12- You get reward.
lvl 15- Full basic charm gear and the ability stay
lvl 23- Ability to use jug pets.
lvl 25- Sic (allows you to use TP attacks of your pets)
lvl 30- tame (allows you to dispel hate on something you mischarmed on (most common application))
lvl 35- Leave and by now, pets will hold hate so you can actually melee with your pets.
lvl 40 - You get sneak from your whm sub.
lvl 42 - Regen and blink from your whm sub.
Those are just a few of the things that you will be looking forward to. Not to mention moving on to new areas with new pets to charm, new equipment to wear, new weaponskills etc.
Yes, WHM is pretty much the end all be all sub for BST. While NIN -does- work, it is difficult to use. Many times in your BST career you'll need cures for status or to take care of damage from a mob (very important when exping near undead). You'll need sneak at some point too, not to mention invis, and Silent oils/Prism powders are expensive as hell. At 50 you get Autoregen (Very awesome to have, even if it isn't a whole lot of HP, it adds up), which will REALLY help you survive sometimes.
With NIN you get a few debuffs and your Utsu, it's really preference, but In my opinion having a WHM sub will make things MUCH safer than using a NIN sub.
A few tips:
1. Try to stay away from parties. Sometimes it isn't avoidable, there are just some levels where you have little choice but to exp near a party. If you are, be prepared for "OMFG BST MPK!!111shift11!!" because it happens all the time. Generally speaking, if a party dies, and you're within a 20 mile radius, it's automatically your fault. Try not to let it get to you.
2. Know your mobs and your area. Know spawn rates of your mobs, what they aggro to. What mobs surround the mobs you're exping on. KNow the terrain in case you have to escape. Always have some sort of contingency plan.
3. 1-10 will be faster on BST if you just sub WAR and melee it out without charming. However, I think this would be a great time to learn how to properly charm and control hate. Use 1-10 as your learning grounds before dying will cost you 1,000's of exp.
4. Know your pets, what intimidates what, what mobs are better than others, and especially what mobs are charm resistant. For instance, Jelly > anythingelse. Jellies hit hard, have good TP attacks, and take virtually no physical damage. However, they are EXTREMELY hard to charm.
5. ALWAYS pay attention. Even if you think you're safe, you're not. Always be looking around, paying attention to your radar.
6. If you have to cast spells, let the pet have hate and walk out of hate range to cast. Generally, hate range is determined by the radar. Once you can no longer see your pet and the mob on the radar, you are safe to cast without pulling hate.
7. Don't melee early on. It's okay to add a smack once in a while to even things up between your pet and the mob, but meleeing with pets the entire battle gets you killed until they start holding hate, which happens around level 35.
This is just stuff from my own experience, there's a lot more to the job, but this is information for you to consider when deciding if you want to play the job. I also recommend going to ffxi.allakhazam.com and looking at the stickies on the BST forums. They're very informative and will help A LOT. Especially the guide to areas and lvls. Good luck.