working with Aer Lingus, you'll be in a very unionised environment, and working basically on 3 aircraft types, 737, a32x and A330, and probably be limited in the things you get to do as a new arrival.
Servisair handle just about anything from Seneca (BAD news if you know what you're doing) to Antonov 124's and everything in between, so you'll get much more variety, but the working conditions are not great, the management are worse, and despite what you may have been told, there's no guarantees of any sort at the end of October, they always let most go, with the option to take some of them back next year.
Most of the guys on the ramp are good to work with, ramp controllers & duty managers have their moments and will shaft you if they can, above them are to be trusted about as far as you'd trust a snake.
Loading FCA 757's is bloody hard work, 6 tonnes off and 6 tonnes on in an hour, and you may well end up handling every bag.
The baggage halls are the pits, hard work, badly designed and laid out, noisy, bad lighting........
You'll get no life at weekends, as a temporary, you'll be rostered in just about every weekend for the summer, and if you want it, there'll be overtime for all the hours that you're not rostered.
Other than that, if you have the choice, the other thing is the money. I don't know what EI are offering, so can't compare it with SA.