Its a no brainer!
The money is less with the start up.(there is a reason for that!)
The risks are higher with the start up.(you are not in a position to gamble)
The problems and frustrations will be higher with a start up company. working for any start up operation in any industry is very hard and involves a lot of flexibilty, do not expect what it says on the tin because everything will change on a daily basis.
The staff and personnel will be friendly in both i'm sure.
Extra aircraft? That wont affect you if they already have an aircraft you fly.
Finally the established operator will look better on your cv and with most of these companies there will probably be the chance to get kept on, even if not you will have a years more experience on type, be able to apply to a wider range of airlines and the recruitment market is on the up.
This is only my advice but hope it helps.
PM me if you want as i suspect i know both airlines and could give you a further insight.