Just found this thread.
BB
Pros
Match Viewer - There's really nothing like this in any other management game I've played. It's really quite enjoyable to find yourself yelling at the screen to tell your PG to pass the rock or your SF to drive the lane.
Training - The quick pace of training ensures you're not waiting 12 weeks or some ridiculous amount of time for a pop on your star player, and the fact that everything (save FT and Stamina) trains more than one skill is genius.
Match Frequency - 2-3 competitive games per week (4 if you include PL) is better than having 1-2.
Player Faces for Non-supporters - no alien-like shadows with eyeballs is a definite plus.
Cons
Player Potentials - Stupid, stupid, stupid. There's already something in place to limit how high a player is trained, it's called his salary. Besides, open-ended games are more fun.
Transfer Search - in HT, players have 8 skills, need 4 at most, and you can search for 4 skills. In BB, players have 12 skills, need 4 at the very least, and you can search for 4. And no search option for experience is crap.
SF Training - No single-position SF training options, and some of the Wingmen trainings are so slow that they're useless.
Hidden Traits - if a player is physical, why not tell me? Better yet, why not make it his specialty and tell me everything it does?
Quick Timeouts - I'll probably be timed out while typing this, so I'll need to copy it, hit send, log in again, open up the forum again, reply to Josef Ka again, change the reply to Everybody, paste, and hit send again. (yep, just checked in another tab, and I've been timed out)
Staff System - Steaming turd. Buy a doctor for $1000, replace him for $1000 in 20 weeks when his salary is too high. Buy a PR guy for a much larger sum, replace him in 15-20 weeks for another larger sum. Scour the staff market for a decently-priced trainer for ten weeks, give up, settle for one a level or two lower just because you can't be bothered with it anymore; repeat next season.
Draft - Random crapshoot.
SCOUT: Hey coach, I just watched this kid play, but I have no idea how tall he is or how old he is, but if I had to rate him, I'd give him 1 out of 5 stars. Here's the boxscore from the match I watched.
COACH: Don't scout that kid again, he's a pile of dung.
SCOUT: I scouted that kid you told me not to again, and I still don't know how tall he is, even though I've watched him play twice, but I now know he's 19 and I think he's got 2-star potential, and I've given him an F grade. You were right, he is a pile of dung.
HT
Pros
Large Userbase - Great community (particularly in the USA) keeps the forums interesting, over 1M matches/week means a great deal of data can be collected in a short time and changes to the match engine/training/attendance/etc can be quickly figured out with some certainty. Also, millions of players means you can usually find what you're looking for when you need it.
Mobile Friendly - nearly all features (the one exception being the new match orders interface) are easily to access via mobile devices, no links hidden in crappy drop-down menus, and matches can be watched live on phones.
Youth - YS or YA, your team gets a new player every week, rather than three pieces of crap at the end of the season.
Match Ratings - Far more meaningful than in BB, and don't require you to read all match reports to scout an opponent.
Forums - well done.
Cons
Training Speed - really slow, particularly for defenders
Boring - particularly for newer users, there's nothing to do but train and lose for the first RL year or so.
Useless Specialties - Technical defender? Quick IM? Powerful FW?