For rats and such a .177 would do fine and at great (airgun) distances such as 50+yds with a good scope. But if all your other guns are .177 then you might want a .22 in your collection. It's up to you. The .177 will take most animals including squirrel & rabbit. With the .22 you can take down racoons, groundhogs, etc.
All Diana rifles shoot very accurately and are some of the finest airguns in the world. I tend to agree with Dave and I think the specs are a typo on the Diana web site for the 460 but all Dave has to do is buy a chrony and tell us what he 's getting (right Dave?)
For rats all you really need is 4-5 ft-lbs of Energy (FPE). The 48 is your best choice out of the 3 Diana Magnums. It doesn't hit or recoil quite as hard as the 350, not sure about the 460 but it's not far in power than the other 2. The 48 still packs 5-10 FPE at 50yds depending what pellets you use and that's very respectable. The 350 has a couple of ft-lbs more, because it carries 30-60fps more velocity with most pellets. See link below.
http://www.straightshooters.com/navagationpages/ourtakselections.html
This above is a collection of performance stats on some of the most popular airgun rifles today, done by 1 on-line vendor and are actually pretty close to what owners are reporting with their own chronometers. The Theoben Eliminator and the Webley Patriot (not tested on that link) are the only 2 Magnum rifles that a little more powerful out of the factory than the RWS 350 (or 460) but cost hundreds $$ more and take a lot of muscle to cock the barrel. They are also hunting rifles like the 350.
The 350 does NOT require 48lbs of force to cock the barrel as Frank said. It only takes 36lbs and this # is accurately reflected in almost every web site I have seen. I own a 350 .22 and I know it's not even close to 40lbs. The Diana 48 may actually require more force to break the barrel than the 350. I have seen 39lbs at some vendors' sites.
To be honest, if all you 're going to do is target shooting and hit a few rats, you might want to look at the new RWS 850 CO2 Magnum in .177. This gun is recoiless and lighter than the Diana sping piston rifles. For target shooting long periods of time, you want a lighter gun that doesn't make you tired and is easy to handle. The 850 takes the large 88gram CO2 bottle (about $6 each or $5 if you buy bulk) and it's good up to 400 shots with a muzzle velocity of ~750fps in .177. No cocking to worry about, and no extensive lube jobs & maintenance like the break-barrel spring piston rifles require. This is not a Diana rifle but is made by Hammerli and marketed by RWS.
As for me I have larger animals to deal with than rats. If they made a 750-800fps CO2 rifle in .22 I 'd grab it in a second. Until that time comes I 'm content with the powerful Diana springers and their pain in the neck dual-recoil action that can eat scopes and mounts for breakfast without proper tuning (extra expense). Good luck with your decision.