Whilst the blade sizes do get bigger as the model numbers increase, the most significant practical difference is in the handle size. I have model numbers 6,8 and 10, all in carbon steel.
The number 6 (the smallest locking Opinel) is very small and light and it tends to sit more in the fingers than the palm when using it. The blade is 73mm long (ie. under three inches) and about 1.5mm thick at the spine. Its essentially unnoticeable when carried in a pocket but I find its a little small for my liking and generally only carry it when use around non-knife people is a serious consideration.
The number 8 blade is 83mm long and about 2mm thick at the spine. The handle sits in the palm better than the 6, but certainly doesn't fill it up. Its probably the best all-round Opinel, in that it is relatively unnoticeable in the pocket and is large enough to give a pretty secure grip.
The number 10 blade is 100mm long and about 2.5mm thick at the spine. This is what the Americans call a "full size" folder, and the handle sits comfortably in my medium sized palm. The 10 has a bit of weight to it and is definitely noticeable, though not uncomfortable, when carried in the pocket. I tend not to carry the 10 around much, but I find it a lovely knife to use for food prep and whitling / wood carving around camp.
I've also handled a number 12, but it felt too big and unwieldy for a folder to me. I'd only recommend the 12 to a bloke with really big hands who absolutely needed a folder rather than a fixed knife.
For 20 quid you could probably get all three, but if you only get two I'd go for the 8 and the 10.
Cheers,
B.