I went through the same thought process a few months ago and opted for the SWAT timed grenades as I play a lot at The Mall and impacts are too restrictive there.
The SWAT is also very light in weight compared to the impacts I saw. This was a big factor as the thrower is responsible for where/who it hits and can face a personal injury claim if injures a player. I walked around a corner at the wrong time and got hit in the shin by a SWAT and hardly felt it, the impact grenade than hit me the previous week left a nice bruise and a fair bit of pain for a few days (I just took this as part of the game and left it at that, other may not have done) so that is worth a thought.
The single shots I have do take a little getting used to to load but I can empty/prime and reload in 1 minute give or take a few seconds, so not really a big issue and I usually do this walking back to re-gen.
The outer case is holding up very well and can be marked with an ID mark using a solder tip to put initials/tag mark so you know it's yours. As said earlier, the amo is cheap as chips. I use 9mm and primers. You can hear a difference and at The Mall both make a decent sound. Outdoors the 9mm does a little better but is it really worth the extra (50 x 9mm=£15-17....100 x primers=£10) I couldn't really say, but when you put these up against the paper/card ones at £2.50-3.50 then you can see where the saving is.
Get one bought, blow sh!t up, have a great game (",)