Hi there Corax,
Nice to see you venture out of the SST forums :lol:
Personally I really like A:2089. There is a lot of crunch in the rules, but it's all good. Sometimes you have to flcik around the book to find the table you need, and some things take a little figuring out, but given that the rules have to deal with so many different scales of combat (Personal, vehicle and MEk) I think they do very well. I don't think the combat system is that hard, but there are more things to consider than just aC, there's Infra Red signatures, speed modifiers, size modifers. But all that crunch is why I like it.
As to how different it is to MEch Warrior, there I cannot help you, I know nothing of MW.
Hope some of that helps.