105 5700 shifters are indexed for 10 speed and will not work properly with a 9 speed RD. Running a double front setup you'll have no chain drop issues using a 105 RD, so I would suggest that instead of the SLX or XT. 

If you're riding offroad with this bike, go 46-36, or if you're more powerful maybe 48-36. 

If you're going to be on harder gravel and pavement, the 50-34 should do fine. 

I've been using an Ultegra 11-speed setup but with a single SRAM 42t in the front (and therefore no front derailleur) and 11-32 in the back, and it's perfect for anything I'm doing with my cross bike. I'll never need more power than 42x11 can give me, and with the SRAM narrow-wide chainring, I haven't dropped a chain yet even when riding through sloppy mud. 

Good luck!

I have been very happy with my budget 1x9 groupset. I use a top of the range KMC 10spd chain (the gold one) on a wide range 9spd Tiagra cassette, with a XT 9spd small cage rear derraillier and Tiagra shifters. At the front I use a Narrow wide chainride specifically for 10spd chains and a pair of second hand ultegra compact cranks.

This has worked so well, it is so tough and has not put a foot wrong in 1000s of bumpy miles. The two drawbacks are the relatively large spaces between gears and the fact that you have to run an inline cable adjuster. Top tip for this set up is keep the cable tension high, the strong spring in the rear mech allows for this and keeps changes positive and crisp.