"So the order should be reversed. Am I right?"

I think so, but not for the reason stated - others have already explained that.

Although the Rx & Tx SBUF buffers are independent, there is still only one Rx SBUF - so as soon as a charcter arrives, you need to get it out as soon as possible to be ready for the next incoming character.
On the other hand, you have control of the Tx side - you can take your time refilling that, if you want.

Therefore, I would say that it's best to handle RI first, as that's the most urgent.
TI can wait.