IMHO: Depends on what parts you are going to put in it, and what you hope to accomplish by using an original receiver gun. If you are going to be putting in a US made barrel, trunnion, FA, etc, there is no reason to do it on either gun, as you've destroyed any originality at that point and defeated the reason why you would use an original receiver. Do it on a PTR, or find one of the FMP/Interport/SLG95 guns or virgin receivers, and do it on one of those. Cheaper starting point, you don't cut up an original gun. If you find one of the Interport receivers, you could probably even mark it "PSG-1" (as it isn't HK), and it doesn't already have a model number on it.
Now, if somehow you have original German PSG parts, do it on the SR9, to maintain all German, just remember you can't legally mark it "PSG-1".