*UNFORTUNATELY THIS IS ALL OUTDATED NOW, SORRY*
BTC Trading Corp. - https://btct.co
Pros: good notifications in email, good analysis of your positions
Cons: always in development, weird UI when new features are added, 24 hour withdrawal limits, but you can request larger amounts but it is delayed a few hours as the admin has to move the bitcoins from cold storage
BitFunder - https://bitfunder.com
Pros: looks more advanced than BTC Trading Corp
Cons: stock prices are typically lower there on cross-listed securities, no email notifications when anything happens, lame we-exchange thing
Havelock Investments - https://havelockinvestments.com
Pros/Cons: Best UI UX, Canada.
MPEx - http://mpex.co
Pros/Cons: Run by a multiple personality lunatic.
Cons: Admission/Listing fees, I forget. Nonexistent UI