I can't comment on the S5 (or any Panasonic) as I've never used one - although I have seen the deals pop up for it occasionally and I do have to admit that my interest has been tickled.
However, FWIW (from a good few years' experience), unless you absolutely need full frame in some sort of professional capacity (and even at that) I have to say it's not 'all that'.
I started out with Canon APS-C and like a lot of others just pretty much had that full frame itch that I eventually did scratch. Used it for around a couple of years then moved to Fuji (and back to APS-C obviously).
For me, unless I started shooting weddings as a professional, which I've little intention on doing, I will never go full frame again. I have garnered much more enjoyment and experience out of the fast affordable primes for Fuji than I ever did for full frame cameras and beefy lenses.
I don't mean to ramble, but if you're looking into full frame for curiosity's sake there may well be cheaper ways to do it. All that said, if it's a hobby and you are keen on branching out then have at it!