I'm surprised no one has mentioned this yet: There are SMBs that have an opening at the bottom that allows you to fill it with air from your 2nd stage regulator (e.g. I fill mine by doing a normal exhale and letting the bubbles from my reg exhaust go into the bottom of my SMB to fill it. Other SMBs are closed at the bottom and have to be filled by either orally inflating it or hooking your LPI hose to it. Some SMBs allow for all 3 inflation methods and some do not. You have to decide what will be best for you.
Also, some SMBs have Solas reflective tape to make them more visible. Some have a loop or clip at the top to allow you to attach a light/beacon. At least one (the DAN SMB) has a strip of radar reflective material up the inside.
Some are orange(-ish) which is supposed to be more visible in daylight (I think) and some are fluorescent yellow, which is supposed to be more visible at night (again, I think).
I've done some diving off the Outer Banks of North Carolina this past summer. On one dive, we had a guy surface a little away from the boat and the current swept him away before he could get to the drag line off the stern. We were in 4 - 5' swells at the time. He did have a safety sausage. By the time the rest of the divers were on the boat and we could pull the anchor and get underway to go retrieve him (on a clear sunny day, by the way), he was somewhere between 1/2 a mile and 3/4 of a mile away and we could only see his SMB when he and our boat both happened to hit the peak of a swell at the same time. If I'm diving out there, I will always carry a big, fat, 6' (or better) SMB.
OTOH, I did a couple of drift dives off Cozumel last December. Conditions there were so smooth that the DM used a skinny, shorter SMB to let the boat know when/where we were coming up and it was fine, I thought. No significant swell and us and the boat drifting more or less together meant not much chance of getting too far from the boat and not much problem with them being able to see the SMB.
So, I'd say what you need depends a lot on how you see yourself diving.