Both good shooting rifles. I'd look at the bore, etc.. It is actually hard to find a K31 with a bad bore, but common to find the stocks all dented up. Matching numbers are usually common on the K31s with the possible exception of the magazine. K31s with walnut stocks tend to be a little more sought after than the beech stocks. Look for the Swiss crest on the right hand side of the butt stock, and on the receiver. Sanded and refinished stocks decrease the value. Some VZ24s have matching numbered stocks, but that is rare. More common to find VZ24s with mismatched parts.
A VZ24 with an intact lion's crest is hard to find, and increases the value. Second would be a numbered crest, and many have had the crests scrubed. A VZ24 with an intact King Carol crest is basically impossible to find, and would be very valuable.
I've got both rifles, and the K31 is the better shooter. GP11 ammo comes and goes, but there was a recent batch released, and prices have stayed around 50 to 55 cents a round. Many consider GP11 ammo to be match grade, and few re-loaders have managed to better it.
The VZ24 is typically 8 mm, and that is pretty easy to find, but typically more expensive. I usually reload my 8 mm rounds.
Can't go wrong with either. A good deal would be under $400 for either.