TT feed is also offered by velocity futures. you may try both the trials and decide what you like more.
I'm with Amp futures, using Ninja+TT. I've never used Zen-fire real-time before, so it's hard to make any reliable comparison. I don't think there is anything lacking in the TT feed. Executions are timely and fills are great (so far in my 2 weeks of futures trading). Choosing Ninja+TT option under Amp is pretty much the same as going with Velocity, only with cheaper commissions. The only downside with Velocity as a clearing house is the daily statement of the day are only published after midnight, which sucks because I've gotta wait till the next day to check if my log entries (which i key in manually) are correct.