What bike are you riding? No ones even asked.
What sort of riding are you doing? Going RTW? Out of EU? S. America? Adventure bike? Dual sport? :confused1:

Tire availability varies round the world. If you're building a Race Bike then the 18" is the better choice. Enduro racers use them for reasons layed out earlier, motocrossers use 19"'s for handling. For travel on a dual sport the common 17" tire is fine ... and IMO, preferred.

In this part of the world (USA, Canada, Latin America) 17" tires are cheaper and more common than 18" tires. I ride a DR650, it uses a 120/17 in stock form. You can go BIGGER (many run a 130). It rides fine with a 130. Both easily available in the Americas.

Both sizes are common are available in street, dual sport and knobby style tires. If you go above a 130 on the DR650, handling goes to Hell. This pretty much true for the KLR, F650 a few other dual sport bikes. A wide tire can ruin handling.

More dirt biased dual sport bikes use 18" rear tires. (XR650, XR400, XR250) Not sure what the Yam's use ... we haven't had XT600 here since 1995, but I think it's a 120 or 130/17, same as KLR or DR650.

Throughout Mexico and Latin America it's easier to find a 17" dual sport rear tire than an 18". You WILL find 18" for small (80cc to 250cc) bikes with narrow rims. Those tire won't fit your wider 17 or 18" rim. Years ago, 18" tires were more common, but no longer the case around here. Maybe in Europe you don't find 17"'s as common dual sport bikes aren't sold there?

As long as you travel where 18"'s are sold you'll be OK. Over here there are not many street biased dual sport tires in 18" size ... and the 18" dirt tires can be quite expensive.

Good luck with the new wheel ... hope it fits on the bike OK.