I prefer a semi-closed circuit bag. Some of them have the option of being filled a BC inflator hose, using a non-locking QD fitting on the SMB. I've used a 9" BC hose on a Mk 10 stage bottle regulator. The hose normally rests along the side of the tank and when launching the SMB, I can flip the hose forward on the rotating turret and then use it to inflate the bag off the remaining gas in my stage bottle with my travel gas in it.
If I'm not carrying a stage, I fill it using my primary second stage. If you've got a closed circuit SMB, that's not an option and you'll need a BC hose with enough length to let you fill the SMB far enough in front of you to prevent any entanglements.
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Divers have been bent like pretzels and killed getting drug up with a lift bag or SMB on dives with significant decompression obligations, so you want to place your primary focus on ensuring nothing snags the SMB or reel during the launch. You also want to be sure that you can release a reel if it jams. I avoid reels with the locking screw on the front as the SMB will snake and corkscrew it's way to the surface and as such the velocity isn't constant. The sudden change in line speed coming off the reel can throw small loops of line that can loop around a front mounted locking screw and jam the reel.
At shallower launch depths a finger spool is a good option, as they are almost jam proof and if they do jam and you release it (or lose your grip on it), the odds are that it will ascend a few feet, clear it self, and then just dance around a bit as the line unspool, allowing you to recover it once the bag is on the surface.
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A 7' , large diameter SMB is nice in rough seas, particularly if you are live boat diving.
However, you need to shoot them from depth and you need to have enough negative buoyancy to be able to hang on it hard enough to keep it vertical once it is on the surface to get full benefit of the large size.
If you fill a 7' 50 pound SMB, like the one sold by XS Scuba, coming off a wreck at 180'-200', you've got the advantage of 6.5-7 ATM to help fill the bag, so you only need to put about 7 pounds of gas in the SMB. That's pretty easy to do OC by just exhaling and using lung volume to maintain neutral buoyancy as the SMB is filled.
If you have the even larger 7', 90 pound bag, you're going to need to put about 13 pounds of gas in it, which will mean finning down as well as exhaling if you want to maintain neutral buoyancy. Finning down can be problematic as it makes it harder to keep the SMB and reel out in front of you and clear of any entanglements. In that case, you might prefer to dump a few pounds of lift from your wing and fin upward until the SMB starts to fill.
If you're launching a 90 pound bag from 100' you've got a significant challenge as you now need to get about 22 pounds of gas in the SMB to fully inflate it, and that's not likely to happen. You're going to have a partially inflated SMB until you reach the surface to finish the job. In this case the narrower 7', 50 pound SMB is a better choice as it will only need a much more manageable 12.5 pounds of gas.
Perfect horizontal trim looks cool, but I'm a practical guy and I have no issues with being at a 45degree up angle or even vertical in mid water to launch a large SMB, particularly when it helps ensure the reel and bag stay un-entangled. It beats trying to look cool in a a hyberbaric chamber.
In either case, once the SMB is on the surface you'll need a fair amount of negative buoyancy to keep it upright on the surface - at least 5 pounds. That's not an issue in technical diving where you'll end the dive with a 1/3rd reserve where you'll have at least 4 pounds of back gas remaining with 18/45, plus a 50% reserve in your deco gas to add some negative buoyancy.
However it can be an issue on a single tank pretty fish dive where having an extra 5-7 pounds of negative buoyancy leaves you over weighted, and in that case the options are to have the SMB laying down on the surface during the safety stop, swim down constantly during the stop, or just let it lie on the surface and get it vertical once you're on the surface.
