I didn't have problems with either so I would probably go with whichever seemed more likely to kill whatever bacteria I was infected with. Keflex is a more hardcore drug that I would expect to mess up my GI more, though.
I won't, personally, take antibiotics in most cases, unless there is a positive bacterial culture for a specific thing. When I am prescribed antibiotics "just because" I let them sit on my shelf for 3 days, and only take them if I am not improving in that timeframe (or if my symptoms become worse or scary). In the 10 years or so since I started doing this, I think I've been prescribed antibiotics twelve times, but I've only actually taken them a couple times.