From what I've read the Advent Vega has lots of probs , poor screens not just viewing angels but not working to touch properly, keys sticking on mains power some people onto their 6/7 due to problems and a new version due soon - lots of refurbs available so suggest that there are inherent issues - 3 month warranty doesn't give me peace of mind I'm afraid.
From my understanding the Archos 10.1 is better build quality than the arnova (achos budget range) and had more flexibility in terms of mods etc than the arnova or at least for now.
It running Android 2.2 and full market is easily added
Had a look at these in store and screen responsiveness was fine and viewing angles nowhere near as bad as what I have read, browsing was quick and a couple of demo games played well with decent sound from speakers.
Full warranty
I'm looking at purchase for the kids, few games, internet and videos (the 10.1 has greater range of file compatibility) so doubt there'll be much multi tasking to worry about