The 5100 has its own problems, but it is much easier to use and takes far less code space. There's tons of examples out there on the web for this device so you should be fine with it. As you go, make sure you can stand a board hanging up or rebooting unexpectedly. Power failures happen and the arduino will hang up from time to time for no apparent reason. The reason is almost certainly bad code, yours or something in a library somewhere, but you may never find it. So, get familiar with the watchdog timer and resetting the time when you have to reboot.
I've played with three different temperature sensors now and the lm34 was the most accurate and easiest to use. So, I'm surprised you are having trouble with the lm35. Could be you're right about the quality, but everything I've gotten out of China has worked first try; lucky I guess.