It depends on how you ride on-road.
If your just using the road wheels, to get down the road and don't need better road holding, the 21" would be just fine. But with only a spare hub, you could go for the 19" front.
However, the Pirelli MT90 is not available with a proper size tire for the 19" front. You would need a 100/90-19 or 110/80-19 front tire. So you would either mix and match tire models/ brands, or look for another brand.
You could definately keep the spare stock 2,50 x 18 rear wheel. You could then go for a MT90 for the rear in 140/80-18 and a Scorpion Trail 100/90-19 for the front. For normal road riding, those sizes would be good enough.
There are a few bikes that come standard with similair front/ back wheels. The advantage of the 19" front would be the availability of more road tires. The 2,50 x 18 rear is a bit more difficult to find road tires for. However, there are still plenty off road tires available:
On the 2,50 x 18 rim, you could also fit a 130 width tire (130/90 or 130/80 or 130/70-18)
With the standard wheels (21/18) you could also mount Heidenau K60 Scout, very good on the road, ok for light off-road.
With the 19" you have even more choice for road oriented tires. But unless you go with a matching wider rear wheel, you might not benefit from the 19" wider front rim for the road. Considering resale of the spare wheels later on, going to a 3.50 x 17 front wheel and using the spare rear wheel to relace that hub to a 5.00x17 wheel, you could have a Supermoto set, for road riding, which are easy to sell again, the 19" front, althoug I really like it, might not be so easy to sell.
The choice for me to go with the 19/17 wider wheels, was to have the choice to mount aggressive tires, that work well both an- and off-road. For pure street riding at moderate pace, the 21/18 are more than adequate, for aggressive road riding, go with 17/17 Supermoto. Or 19/17 with sport-touring tires.
Good luck with your choice.
Suzuki DR 650 SE 2006
Thanas screwed with this post 06-21-2012 at 12:34 PM