I have both of these rifles, and to me, the K31 is the better rifle, both from quality of tooling and construction, and for accuracy. The 7.5mm Swiss round is readily available from online dealers. Cabela's stocks the round in the Hornady loading. Prvi Partisan also makes the round, it's available online at a good price, and the brass is boxer-primed and reloadable.
8mm Mauser ammo, the cheap surplus is gone. Prvi Partisan makes 8mm Mauser in boxer-priming, also, about the same cost as the 7.5mm Swiss. The days of cheap ammo are over, it's not going to come back, unless it's 7.62x54R.
If cost of the ammo is a concern, both are about equal in commercial loadings. There is more surplus Swiss GP11 ammo available than there is surplus 8mm Mauser, even including the recent Iranian trash that's on the market.