From someone who has looked into this topic extensively, let me tell you that Rithmic and theomne represent and excellent value here. If you are concerned about risk checks, they have a system that sends a risk check to AMP prior to you making a trade, gets clearance, and then allows you to place the trade later without going through risk. I've never used it, just had it mentioned to me by the Rithmic staff, who are quite good.
If you wanna get the lower latency without DMA, your best option is probably TT API with the gateway installed on your own server. If you're not attempting HFT and just want good uptime, I see no need for this as it's far more expensive than the very good value Rithmic provides. Don't expect to be first to the party with either of these solutions though.
Good luck.