I've owned both, either will work. You really can't go wrong with either, but you may have a hell of a time finding a Retro.
The Artist is warmer with a tad more in the low mids.
The Retro, (IMO), is tuned more to the high mids. Both are aggressive, the Artist is just a bit more polished. Both do the hot-rodded Marshall nicely, the Retro is a little more gritty, tad more compressed, (both in a good way). Honestly, I'd have a hard time choosing between the two. The Artist is pretty well known, and the Retro is highly underrated. It's a kick ass amp in its own right.
This video sealed the deal for me when I bought a Retro.