Well here's my 2 cents.
I've never owned a Nissen, but have spoken with more than one person who has. None felt the quality measured up to the original Behr. In one case, temp. gauge readings went up a bit. I suspect the capacity dimished somewhat in that particular case.
If I were in the market for a new radiator, it would choose Behr.
On a side note, I recently purchased a Wahler thermostat over the WEB, so I didn't get to see what I was buying up front. The construction, finish, etc. is VERY cheap compared to a Behr thermostat.
When it comes to cooling system parts for my MB, there's one company - Behr.