Having both myself, I'd go for the UMP conversion. Cheaper, looks more like the real thing since you don't have to deal with the extra receiver length, and if you don't plan to SBR it, it's easier to find the US compliance parts. Plus, you don't have to cut a hole in the side of the receiver like you do with a SL8-1 or -6.
As far as rifles go, with how many HK91's and clones you can find out there, unless you are totally in love with .223 you can get a 91 in .308 for a lot less money/hassle than getting a G36 conversion.
Get the UMP, then get a 91 for your long-range shooting.
Where do you do most of your shooting? Indoor Range or Outdoors?
This would be the deciding factor for me as I have more opportunities to shoot at an indoor range so I would go with the UMP since it is .45 caliber. Also, I have a USP45F so I would only have to buy one caliber of ammo.
I like the look of the black USC/UMP but I'm not crazy about the 16" barrel sticking out there. Any chance you will be changing it to an SBR?
Everyone stop talking about the black USC/UMP, I love .45ACP and I'm starting to want one but I have two kids which equals no money for frivolous items.
A forum dedicated to and laser-focused on Heckler & Koch firearms, Heckler & Koch accessories and the owners and enthusiasts that love them! Come join the discussion regarding HK pistols, long arms, NFA arms, HK accessories, HK history and trivia and more.