So, since I own both..
The ES-8 is pretty old now (and a hell of a lot cheaper) and has gone a few years since the last update. I never tried to use the online editor since you practically need the Roland Mini connector to connect to it. Set up is pretty easy, its IMO an easier unit to get started with and Prior to the G3 line was far more capable on the midi front. I have not sold my ES-8, if I can figure out how to get 2 signals into it and both run, Ive got a pretty great idea on another use for it.
The Gigrig stuff.. is a good bit more expensive, but its also a hell of a lot more of a unit, its not as straight forward as the ES 8, but once you get the hang of it, its quicker and easier to build patches and use on the fly, they have put a lot of work in on the midi front and made significant improvements over the G2. G3 has programable buffer per preset so if you want to use buffer sensitive pedals like the Fuzzface, then the G3 is leagues ahead in that department. For your use case, the G3 will support the 2 amps better since it has a selectable phase shift that I believe operates per preset, the ES-8 you will need an external device to do that. The G3 has isolated ground for the second amp to to help eliminate a ground loop buzz from the added amp. You can edit the G3 with bluetooth connection as well, and it has received significantly more updates in its shorter lifespan than the ES8.
There are 2 versions of the G3, the G3 and the G3S.. the S has scribble strips to me I value that, but there is a top plate add on that you can pick up and swap out from the none scribble strip to the scribble strip version, but it doesnt show up often from Gigrig.
Size wise, they are comparable, I wont be home until this weekend I can check then if you would like but.. the G3 is slightly deeper, and slightly wider, I think around 1/2" deeper and maybe 1" wider.. so space wise its about the same. There are more loops on the G3 than ES8, but the jacks are closer together on the G3, I use George L's and it while they fit, it was a little tighter plugging everything up. Straight plugs would have worked a lot better, but I already had the L's.
The one thing a lot of people fail to mention.. budget a couple hundred bucks for cabeling.
Powerwise.. both can be ran from power supplies provided they have a 500+Ma tap.. the G3 likes 24v IIRC...
I did end up going with the Gigrig GenX14 power supply, since I was going to have to get a bigger supply anyway
https://shop.thegigrig.com/products/gen-x-14
The bigger the project the more I would lean towards the G3.
