I like simpler amps, and love the Shiva 20th, but the JP2C is a badass amp too. You're comparing two very, very different amps. Both are very versatile. Both have incredible cleans. The JP2C is more versatile in range of cruch to distorted, but the Shiva 20th in modern mode and boosted can do anything without a pedal. The Shiva 20th has a mid shift which scoops out the mids and lends it more towards metal sounds, but of course the JP2C has sliders and all kinds of other options. Mesa includes power options which are very nice to have, but the Shiva 20th sounds good at any volume. The main benefit of the Mesa power settings aren't for getting lower volumes in my opinions. 5w mode doesn't sound good to me unless you're cranking the clean channel or something.
Instead of weighing a deal between two amps, just look for an amp which suits you the most. You're not going to get a good comparison between these two because they're just too different. Honestly man, after thousands and thousands of wasted dollars flipping gear, it really comes down to what suits you and how you like to use it. Don't waste money buying gear to try out because you'll just waste money, and you'll waste time that could be spent crafting your sound on an amp you know you'll like.
