ON BIKES ...
For touring the West and SouthWest of the USA I'm not a fan of 200 or 250cc bikes. It's a HUGE region and typical speeds (even on small two lane roads) are 70 to 80 MPH or higher. You will also encounter a lot of HP sapping mountains.
You'll need at least a 200 mile range.

You certainly could DO IT on a DR200 ... but of all Suzuki dual sports (which I know fairly well) the DR200 is my least favorite. Not a bad bike, fairly tough and reliable. But underpowered and under suspended and not very well supported in the after market compared to DR650, KLR, XR650L Honda. I can't imagine crossing Nevada or Utah on a DR200. (I did it on a DRZ400E and that nearly killed me! )

I like the comfort, space, ease of packing and the easy road speed of the 650 class bikes. You DO pay a bit for this in more fuel, but fuel here is at a 15 year low ... it's cheaper than Mexico!

Unless you really search around for alternate routes (or dirt routes), the main roads in N. Mexico are pretty fast paced. Once on dirt roads on out in the Desert you would be fine on the 200 or 250. But with a 650 you can really cover ground if required and a well set up 650 will do deep sand and rock quite well.

I'm a long time DR650 owner (65K miles on current bike). Also owned nearly all 650 competition (KLR, XR650) and ridden the rest (BMW, KTM, Husky).
Bang for buck and fantastic versatility make the DR650 hard to beat. Tens of thousands of owners can confirm this.

But go with your heart.

If you buy a DR650 try to find one with suspension and seat done. Both top my list for mods. Good luck shopping ... several DR650 guys here, any questions, just ask!