My feelings, right or wrong, is that Northern tends to get the better pick of the lot regarding the merchandise that overlap between HF and Northern. You pay a bit more, usually, at Northern, but it seems that one is much less likely to get a dud from Northern than HF, though both have that chance with respect to their house brands. Regardless of where you buy, it often helps to smooth rough edges and tear the tools apart to clean out any manufacturing waste or burrs -- which can greatly improve the tool.
That said, I don't normally buy power tools from either as I want my tools to just work...and last...for years without a lot of hassle. But other odds an ends I will occasionally buy from Northern.
That said, if it is too cheap then I would pass, the last thing one wants to deal with are bad air hoses or leaking fittings on a retractable real -- such things always seem to fail right when you most need them!