My Cantabile PC is a Gigabyte Brix i7 4770R, with 16GB RAM and a 1TB SSD, and it runs beautifully, including with Diva (although I still do the jBridge "trick" just to be safe - then the load meter is hardly affected). Up until recently I was using a standard old 1280x1024 LCD desktop monitor, but was inspired by this thread to get something nicer, namely a GeChic 1303I, which is a beautiful 1920 x 1080 (native) touch screen. I thought it was a bit expensive, but once I had it I was converted! It's great with Cantabile, and particularly good for displaying music scores clearly.
I definately prefer having a PC module in the rack, rather than a laptop. It feels more slick, and I can have important cabling plumbed in the rack instead of trailing around the stage. I use an Akai LPD8 to control Cantabile (state/song next/prev, toggling live mode, various other things), so don't need access to the PC keyboard during the gig (although typically for next state I use a foot switch - I don't generally have a hand free to be pressing buttons like @Torsten's famous Big Red Button™ to go to the next state!). And now with the touch screen I can also use the controller bar at the bottom of Cantabile for any interaction during the gig too, which is nice.