There are quite a few differences.

SRT8 can be optioned with the Kicker sound system
Both Auto and Manual SRT8 get limited slip differential. Only manual R/T gets it.
SRT8 has the hood stripes, R/T has the hood-to-fender stripes
SRT8 has more aggressively bolstered seats
SRT8 has much better brakes, brembo brakes
SRT8 rear spoiler is black regardless of paint color, R/T is always color matched

I am sure there is much more that I am forgetting. It will take a lot more than $2k to get to the same features as the SRT8.

Then there is always the argument that any performance upgrades you make to the R/T can be made to the SRT8 to make it that much faster as well.

Only you can decide what is best for you. However, if you are even considering an SRT8 you might want to take it into serious consideration. If you check Charger forums and such you can see that there are a lot of people who were on the fence about R/T and SRT8, got the R/T, and later decided they HAD to have the SRT8 and were still upside down on their loan. The R/T is NOT a bad car, but it is neither an SRT8. You need to decide for yourself what you really want and can really afford.