Alright two predominately anabolic compounds
1) strength: Deca is more powerful mg for mg compared to primo, but most guys won't use more than 400-600mg a week, where as you could use tons of primo (1+ gm) so primo could be more powerful
2)sides: Deca can cause gyno and some bloat while primo won't cause either (probably the closest we'll ever have to a side effect free safe steroid); both easy on hair, prostate, etc...
3)suppresiveness: Deca is VERY supressive while primo isn't very supressive
4)cost: Deca is pretty inexpensive (depends on brand really), primo is VERY pricey
5)concentration: deca is usually more concentrated (although it ranges from 50-300mg/mL), primo is never more than 100mg/mL; so since more primo is needed to match the anabolic effects of deca and it's usually less concentrated, much higher injection volumes are needed
6)availability: deca is more readily available, but finding primo is becoming much easier as of late w/ ttokkyo and RSoC now making it
7)other:deca helps some peoples joint feel better, primo comes in tabs as well as primo depot
So primo is a better more useful steroid IMO, but just too expensive to use (for poor me and most guys) unless it's used for a couple weeks as a light bridge. Deca is a good to great (everyone's different) bulking steroid for some, but in many people can't use it b/c it causes unwanted side effects (namely gyno, deca dick, or very difficult HPTA recovery).
Let me know if I missed something guys.