Out of whats been mentioned, the A70 is definitely, without a shred of doubt the best camera. The 3100 has a serious problem with battery life. It is a compact camera but with the amount of batteries you'll need to carry, might as well get the bigger a70.
If you want something smaller, the sony P8 has a similar spec and uses li-ion batteries, giving great batterylife. It also takes long movies, if thats relevatn to you. It uses the more expensive memory stick though.
<b><font color=red>They that can give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety.</font color=red></b>