Atma would be the clear winner for me between these two amps.
It gives you three distinct and useful circuits, and the ability to modify them into something that suits your personal tastes. The "70s/Vintage" setting is an excellent Plexi replicator. There is a "why did you get rid of" thread that lists "Too Modern" as the biggest reason for moving-on.
It also has the capability to go from bedroom (1w) to screaming loud at 18w.
Weaknesses? Changing tubes in the combo seems to be an unpleasant procedure. You can't access the tubes unless you take the amp apart. And...it POPS like a mofo when you power-off. Although not bad for the amp, it seems like a backfiring jalopy in car terms. I don't think a $2000 amp should do this, period.
I've actually owned the Mercury twice, so it's no dog. I used both KT66 and EL34 tubes. I think it has it's own tonal thing going (don't expect a Marshall clone). It also makes a hell of a lot of noise on the third boost position. When you're playing at low volumes you hear equal parts amp hiss and actual guitar tone. This was a show killer for me both times I owned one. It was one of the first boutique circuits with attenuation (that I recall), but I think there are better choices in today's market.
Here's the official description of the new Mercury V that's going to be released in about a month.
"The new Carr Mercury V is the latest offering from Carr Amplification, and is the older British cousin of their stellar Skylark amp.
This new version of the much loved Mercury comes with a highly evolved feature set borrowing the best of the past while rocketing into the future. The Mercury V is highly touch sensitive with a huge range of tones from 60s aggressive clean through 70s rock into 80s and 90s tasty crunch.
Carr's built in attenuator delivers real power tube overdrive at whisper levels retaining the feel and juice of full volume fun. Like recording on your computer? Just plug the Mercury V line out into your A/D interface and go! The Luscious Spring Reverb, full ranging Overdrive options, and effective tone controls make the Mercury V an invaluable recording tool. Home player? Experience big amp satisfaction with domestic tranquility. Gigging musician? 16 burley watts in full power means the Mercury V is a commanding stage amp. Mercury V wins the six string triathlon: Home, Studio, and Stage! Plus, you get everything Carr is known for. In-house crafted solid pine dovetailed cabinets, bleeding edge power supply components mixed with Mid-Century signal elements, and true point to point hand wiring."