Difference between a R and T are not so big, but together they are much more money to spend on upgrade then replacement.
I would aways sell the T and buy an R.
But the R, assume you mean the FW-05R, is old model, current model is FW-05RR, this has even more differences from the T. Way too expensive to upgrade a T to an RR !
What do you get with the RR ? Alu schoks, stabs, cabon parts, V-One rear, better steering due to better blocks in front, centax clutch, better chassis and much more !
But be aware that an RR is a completely other car then the T, mostly because you normally put a much faster engine in the RR. Stock Kyosho is 0.5hp... a affordable good engine brings 1.2HP or more on the table !
Other accupack, better steering servo and startbox needed ! (Pullstart doesn't fit without modification of the enginestands of the RR)
The RR is a great car, depending on track it can be faster or slower (very little difference) the a V-One RRR WCE !
And last but not least... the RR only comes as a chassis kit ! So nothing like reciever, servo's, engine, muffler, body included !!