Rain doesn't damage a bike as long as you keep it clean and lubricated. Single speeds are fine for commuting esp. in a city as flat as Houston. Neither bike, though, is optimal for commuting. MTBs can make pretty decent commuters once you add slicks, fenders, etc. but a suspension fork isn't great for commuting. The Kilo probably won't take a fat tire and fender.

It's N + 1 time. Houston has a good CL. Find a vintage road bike with center pulls or long reach side pulls. Then you can fit a 700 x 28c and fenders. You can ride it geared or single speed/fixed. Plus you won't care what the paint job looks like within reason as long as you can make the bike mechanically sound.