Okay the Harrier is young, but it handles, it's fast enough to be entertaining, it's only going to get better. And when it can use it's targeting pod, you'll be able to carry a shitload of GBU-12s.
The problem with the A10 for me is it's a flying computer. You actually fly it is boring, but it's also a huge learning curve. It's very very effective, yes, but personally the Harrier wins, and not just because it's new.
I also like the Viggen however the required level of pre-planning for multiplayer public servers is frustrating.