What format? 135, 120, large? Lower light or daytime?
Of the two I'd tend to go 400 on 120 and 160 on 135, purely for the shutter speeds. The look is almost the same, I do like 160 a bit more but they are very similar. I actually prefer Portra 800 out of the lot but if cost is an issue there are other options, as others said. Gold (not a fan), 400D (nice film but the colours are different).
Also, do you scan yourself or send it to a lab? If you scan yourself just pick the ISO that works for you and the rest can be made to look like anything.