My view was with Primo, is that you will keep more of the LBM gains that you made. I'm trying like crazy to find the reference material i read that in, but for now just treat it like my impression or viewpoint until i can get the sources to post up.
I'm very unsure of Tren, as I haven't looked into it personally, but I was almost positive the main benefit of Primo was that you kept a larger portion of your gains over most any other AAS. So the difference i see if they can both be sufficient in this role, but with Primo you may not see the explosive growth that you get with others, but should have a better retention.
I hope this makes sense, but if it doesn't, remember Corn hasn't had his coffee yet