The MP5 clones can be had in the low $1K range.
Speaking from experience, my USC/UMP conversion ended up a little over $3K (that's with case, tac light, optics etc).
The USC/UMP's big advantage over the MP5 clones is that it's genuine H&K, but you pay accordingly.
If money is no object, then I'd say get both. If you gotta pick one, I'd go with the MP5 clone. In the long run if you really get into it and want to take the hobby to the next level, you can always find a pre-86 full-auto lower for it (again, priced accordingly), where you can't do that for the UMP.