Something no one has mentioned -- the 5th Gen gets better gas mileage than the 4th Gen V8. I don't know your weekly/monthly budget for gasoline, but gas prices are creeping up, and forecast to continue higher. That is the main reason that we recently bought a 4th Gen V6 as opposed to the 4th Gen V8. So if you aren't really attached to the torque of the V8, you might want to think a year or two or three down the road about operating expenses, in which case, the 5th Gen gets the edge in your sweepstakes on that particular issue.
As I said, we own a 4th Gen, but that's basically because we don't buy new cars. I expect we will drive this 2-3 years and probably buy a used 5th Gen. I like the looks of the 4th Gen from the front, but not much from the rear, and I hate the stupid factory front end rake. I prefer the looks of the 5th Gen, and the additional interior space. But bottom line, I don't buy Toyota's for looks, I buy them because of the quality behind that nameplate. If I happen to like the looks, it's just a bonus, LOL.
I agree with others who have said that the Torsen Differential of the 4th Gen is most definitely superior to the 4WD system in the 5th Gen SR5, but for our usage of 4WD, it's not a deal breaker. We still expect to move up to a used 5th Gen a few years hence. We rented one for a long weekend, and really liked it.
Ultimately, your decision between the two vehicles will depend on your personal tastes and usage. But I agree with someone who said that 20K less miles and a newer truck for $2K more is a no-brainer. It would be for me if I were in your shoes. Good luck!