Don't listen to these guys, there's a few people on BSN with a lot of creds and actually decided to run some tests about UR drops and Arsenal / Reserves packs yielded almost 2x the amount of UR's as PSP's did.
I'm sure I can find about a dozen threads on BSN about it but I'm too lazy, if it's that important go find them for yourself.
So Arsenal Packs are indeed better for UR weapons?
Yes. I think it might also be effected by how many cards you have maxed out too. I know once I got within like 10 cards away from having all my rare guns maxed, Arsenal Packs started spitting out UR's every 5 packs or so.