1.2 may be more stable for you, and a better choice if you're just starting out. If you want to change to 1.2 though, your best bet is to reinstall with the 'show pre-release versions' option off in SDC, as directly downgrading can cause problems.

The 1.3 beta patches are intended solely for those willing to risk more problems from the new and untested changes, but if you have no problems with it, there is no real reason you shouldn't use it. Just be fair-warned that problems can and likely will occur with these (as with any betas) patches.

Really there isn't much difference between update1.2 and 1.3beta2a. The extra round of ship moves with a save/load has been removed with 1.3 beta which is nice. Noticed a few small bugs fixed like ships no longer getting stuck if moving next to a ship being upgraded. Haven't had any really big issues with 1.3beta but did have my starting colony ship just disappear a few turns into the current game I'm playing but I just continued on and am doing fine. Not sure but sort of looks like they upped the ship upgrade costs with the 1.3beta.

Also the AI has been improved quite a bit with the 1.3beta, especially in the planet development department. And there are more options in the difficulty settings with 1.3 beta especially between tough and suicidal or so I've read.

Personally I feel update1.2 is a very good game and worth playing for historical reasons if nothing else.

Either way you go, sure you'll have a great time -- get ready to be hooked bad.