I would go with the shorter TT and compensate with a longer stem to get the required reach.
You can't really compare top tube lengths unless you're also comparing seat tube angles. Best to compare the "reach" of the frames (measuring from a plumb line dropped through the BB forward to the center of the headtube/toptube junction).
But, if everything else is equal AND you have enough standover, I'd go with the longer toptube/shorter stem approach. The shorter toptube/longer stem may put too much weight over the front end AND cause problems with toe overlap.
Scott is totally dead on here - often TT length gets over-simplified but 1 degree difference is essentially 1-2cm difference in distance from the HT to your seat post, depending on frame size and seat post extension. Not many people realize that. Everyone used to always say Lemond Poprads were so long but not if you set your seat relative to the bottom bracket (as you should).
I would also add in that it depends on what overall bike sizes you're looking at. On small sizes you might just want more room so you can shoulder it easier. I'd make sure you you have at least the bare minimum standover and then see which can put you closer to your road position (if you have a road bike, and feel that position is dialed). Then maybe shorten the stem a tad, or raise it, if you rather be more upright for off-road riding.
Good luck, hope that helps.
It also depends on how the frame was designed to ride. I could be misquoting here, but I *think* I recall Adam Myerson saying that the Stevens were designed for a longer stem to put the rider where they should be. But I'm going off the top of my head so don't take that as gospel.
Next question I am moving up to a carbon bike this year, of course each retailer thinks there's is the best but I have narrowed it down to: Ridley X-fire, Cannondale SuperX 4 and Redline conquest carbon pro. Wheelset that comes with each one irrelevant because I am switching them out. Don't know any non bias riders personally but what little reviews that seems to be out there, they do point to the Redline.
I've also read really good reviews about the Redline Carbon bikes, but I just built up a Felt F1x frame and absolutely love it, so I have to plug their F3 bike (Felt runs the same carbon frame throughout their entire carbon line unlike Ridley and Cannondale) as another bike to throw into the mix.
Be careful when choosing a size between the three frames that you listed; there is a 4 in difference between the stand-over height of a 54 cm Ridley X-Fire and a 54cm Redline Conquest Carbon. Also, are you Shimano or Sram fan? Unless you have a full groupset laying around, the component set of the bike may help you choose the "right" bike.
As a side note - the X-Fire is also missing water cage bosses. Not sure if that's a deal breaker or not, but it was a hassle on my last bike and depending on your intentions, it could become a hassle for you too (unless you like to lug a CamleBak everywhere you go.)
Good luck on the search :)
Love the sram, Competitive cyclist does a sram build on the xfire which makes them all equal in the group department, except the Redline is a mix of rival and apex but I don't know how much of a negative that is. It's hard to judge a cross bike just by test riding it around some neighborhoods so all the responses are important. I guess I just want the best bang for the buck, my strength is in the technical sections, so I am not sure if I get a frame that plays to that or the opposite.
I'm with you - SRAM and technical. The bike that my Felt replaced (Tommaso Diavolo) was designed much like a Ridley (traditional open triangle with straight TT) and I must say that when comparing the two bikes, I'm happier with a smaller snappier frame (plus I'm a small guy with a short inseam so....)
I haven't used SRAM's Apex, but I have quite a bit of experience with Rival and think (IMHO) that it's the best group for the money in the SRAM line (especially for CX) and the shifters are a better fit for my hands.
Plus, the Redline's internal cabling routing is sweet; my Tommaso is set up that way and it's the one difference between the Diavolo and the F1X that I really miss (unless I go with Di2 and that's never going to happen.)