I wouldn't think that the computer or the tape deck would have any preference between the external loop and the tape loop - the two are doing basically the same thing for recording purposes (a fixed level output). It might depend more than anything else on how you use the two recording devices. If you record to the tape deck a lot while listening to some other source, then the tape deck would be better off connected to the tape loop instead of the processor loop. On the other hand, if you really do think that you're not going to record to tape much anymore, you may want to hook it to the processor loop as a sort of insurance policy ("I don't think I'll need it, but I don't want to have to run extra cables to do it if the need does arise") and let the computer have the tape loop for the more routine recording duties.