You are getting pretty good advice from several people; TT is hands down best platform for pro traders/execution/hedge trading and great with writing code etc... and is a very "pro" platform, I agreed about meta a few comments back.
I will say (as a professional trader) $500 is not only very expensive and I mean expensive, but if you think you have something worth that then you keep it yourself and run it and crush it. Also I don't know too many hedge fund owners or high net worth traders or even just good traders that would pay that b/c they know how to trade and have their own thing/model etc... but now lets try to imagine the rest of particpants paying $500/month.
I see this as a service, lower price-take it everyone (platform) to buy like tradestation and go for volume on your sales.
*I skimmed through this discussion so I apologize if I brought something up that had already been deemed decided.
Oh and I agree with I believe "bizros" about metatrader especiallly with launch of futures trading in 5, my fund actually opened an account with it and great execution so I see success.
Best platforms (even if 1/2 a broker)
TT
CQG and data
TWS by IB
MarketDElta especially now after merging the trading part with CQG platform and CQG IG for $199/month
QST-depending on style
CTS
OEC
Multi