both are the same, you don't get any performance loss (at least nothing major) by going modular, but you get the added benefit of only having to have the cables you use in the case, making cable management much simpler.
The 850HX seems to be on a new platform. Its gold certified by 80+ even though Corsair only labels it at silver. Difference is 80+ only test at 25 degrees C and Corsair ran theirs up to 40 or 50. What better efficiency means is it will use less power from the wall to produce the same amount for the PC. If you leave it on 24/7, go for the HX, you'll make that extra cost up in electricity bill savings. Also, better efficiency generates less heat in the process. That usually leads to longer component life, but I'd still expect to get 8 years out of the TX.
You can't go wrong with either, but the HX does edge out the TX for best of the best.
Hit the PSU link in my sig and theres reviews to both in the 55-74amp section.
Well...both are pretty much the same...the one thats modular will allow for better cable managment and tidyness in your case, however if your case has good cable management and lots of space then it doesn't really matter. Both will take up about the same amount of power and produce power output + or - depending on efficiency.