Not many Kilo users on the forum but I love mine. Ive never used the Mesa so I cant comment on it but I did sell my Ampeg and kept the Kilo. The Kilo does have an issue with running bridged mono though.
Jackson PS5 - Jackson CMG - Jackson JS2 KellyBird - Jackson Concert V - Hartke Kilo - Hydrive810 Into The Coven
I haven't played either but I have played an M6 and it was totally badass. It was through an Ampeg 810 and it sounded great. Mesas have great sound and a reputation to back it up, and they hold their resale value much more than Hartke. I would go Mesa.