I don't really know dude. In my opinion they are both really good but P90 doesn't need attachments to be very good. Scar-H needs attachments (which are pretty costly) to be accurate but with the best attachments it has 280 range.
P90 is nuts. It outranks the M249 with a much higher DPS, quicker reload time, and way better accuracy.
Personally, a weapon's only cons shouldn't be things like "expensive," or "high-ranked," or "no attachments." Those aren't cons. The former 2 don't affect you once you have it, and a lack of attachments just means no customization of the stats.