Since it's a 40mm on your Omega, I'd go with Medium rings.

Ring height is different for these reasons:

1) Different objective diameters require different heights for clearance, i.e. you won't be able to mount a 52mm glass in low rings because the objective bell will be pushing into the bbl.

2) Different people hold guns differently, I've seen some people use high mounts with small diameter glasses because of the way they position their head on the stock, some stocks make it hard to get down low enough to use your glass, and some people position their head more upright than others.

SO, find the minimum height that will allow your glass to mount, and then try it on, if it feels too low, then take them back and get a higher set, if it's too high, add a cheek piece to your stock.