It depends on the twist of your barrel, however, because it sounds like your going to buy a factory rifle the twist rates for the 204 and the 22-250 will be set up with slower twist barrels for the lighter bullets. So, that being the case, I would highly recommend you go with the 204 Ruger.
For example lets say we are using the same grain bullet, a 40grn V-Max and a 26" barrel out of both rifles. Both rifles will shoot both bullets at a velocity of 4100fps. However, the 204. 20 cal 40grn bullet have a BC of .275 and the 22-250 .22 cal 40grn bullet has a BC of .200. I think it's pretty obvious which rifle will shoot flatter, it's the 204 hands down. Even if the 22-250 moves up to a 50grn V-Max, that bullet has a BC of .242, so even the heavier .22 cal 50grn bullet still has less BC than the 204 20 cal 40grn Bullet.
The bottom line, the 204, using the 40grn V-max will out shoot and shoot flatter than the 22-250 using either the 40grn, 50 or 55grn .22 cal bullets.
For example here is the MOA adjustments in Elevation and Windage at 500 yards for the 204 Ruger and the 22-250 using the 40grn V-Max at 4100fps;
204 Ruger at 500 yards: Elevation Adjustment-5.25
Windage Adjustment-4.25
22-250 at 500 yards: Elevation Adjustment-6.75
Windage Adjustment-6.75
Because the 22-250 holds more powder I ran the numbers for the 22-250 shooting the 40grn V-Max at 4400fps, which would be a hot load, here the adjustments at 500 yards with that velocity;
22-250 at 500 yards, 40grn V-Max Velocity 4400fps;
Elevation Adjustment-5.75
Windage Adjustment-6.25
The above numbers proof the 204 will out shoot the 22-250 even if it shoots the same bullet 300fps faster. I used my Nightforce Ballistics Program to run the numbers. It is pretty obvious that the 204 Ruger is the flatter shooting rifle when using the light varmint bullets.
However, if the 22-250 is set up with a fast twist barrel for shooting bullets like the 75grn A-Max or 80grn Mk, than the 22-250 will easily out shoot the 204. But in your situation this will not be the case because most all factory 22-250's will have a 1-12 twist barrel. So, that means the 204 Ruger will out shoot the 22-250 at the longer distances.
The bottom line, go with the 204 Ruger. If you reload, you might want to look at the Savage LRPV or the new Savage F-Class style rifles chambered in 6BR. If you want the ultimate varmint cartridge, the 6BR is the way to go. The new 6BR Savage F-Class has 1-8 twist barrel. The great thing about the 6BR, with a 1-8 twist, you can shoot the lighter varmint bullets like the 70grn BT and 75grn V-Max along with VLD type bullets, too. The 6BR will shoot the 75grn V-Max,BC .330) at 3500fps and that combo will easily out shoot both the 204 and the 22-250.
Let us know what you decide to go with. If it comes down to either the 204 or 22-250, the 204 is the better of the two.