I might be able to provide a point of reference for you. Though I haven't heard them both in my system, a friend of mine has a similar system to mine (both using line arrays with digital amplification) and is using a DEQX, while I'm using Tact equipment.
They both are great units and do a very good job of digital room-correction, and both maintain a high degree of transparency. Neither of us would be without them in our systems. The DEQX has the added benefit of speaker correction; a nice addition, depending on the design of your speaker. But the DEQX is considerably more complicated to use.
The Tact is easier to set up and you can see and easily reference the target curve corrections along with the original room measurements, on the computer screen.
The Tact (2.2x--much better than the 2.0) also provides nine immediately available target curves accessible from the remote, to be used for different recordings. I find this to be a great asset, sort of a modern digital tube-rolling kind of thing.
I've used corner placed subwoofers and now use Linkwitz-designed dipole subs (as does my friend) and the ability to time-align and channel balance the subs and integrate them with the mains is simply fabulous. I've never heard such great bass, clean and tight and kept where it belongs.
It can take some getting used to, though. An audiophile friend who owns electrostats came over and was underwhelmed, feeling a lack of bass. I think he was having a hard time adjusting to such clean bass that didn't boom and slop all over the place. With time, it's hard to go back, and I don't really enjoy listening to uncorrected bass anymore.
They are both worthy contenders. If you want speaker correction and are pretty tech-savvy, I'd consider the DEQX. If you want more ease of use and user-friendly features, go with the Tact. I haven't heard the Rives.
