jackrlehman :
i3, but I highly suggest you think about a I5 4460, it is 70$ more but will future proof your PC for any GPU you want to add in the future.
$70 is a lot though for a budget build, and even if you could use 100% of both CPU's the i5-4460 would only be about 25% faster.
The i3-6100 has a newer architecture and faster clock speed which helps it beat the i5-4460 in many scenarios. Despite only two cores, the hyperthreading (and again arch/faster speed) helps it get about 80% of the performance of a full load situation compared to the i5-4460 but that's actually pretty rare.
While I do recommend a true quad-core, it boils down to the budget. I do think it would be a good idea to get an i5-6500 though, even if he has to wait a bit...