Welcome to our friendly forum.
As to your question...They're both fine radar detectors and will provide more the adequate protection, given your feedback.
I've driven with a lot of different radar detectors over the years and I have come to appreciate the differences in operation of most of them.
I think it basically comes down to personality...and which personality you like.
The Rev C. version of either the Whistler Pro 78
or Whistler XTR-690
is especially good and will be the version you receive if you were to purchase one today. Whistler will be offering their new Whistler XTR-695
which will provide features that are unique and may be worth the extra wait. Ka-band reception is especially good with each of the Whistlers. From a "feature" standpoint, these Whistlers have it all over the V955.
The Vector 955, Vector 965, and the Vector 995 are all solid radar detectors across all bands. But I would suggest that you avoid the Vector 940--for the little extra, the V955 is a terrific starting radar detector in the Beltronics Vector series.
You really can't go wrong with either of these choices...they're each very good in their own ways.
If you haven't seen our review of the Whistler XTR-695, the link follows. I've been fortunate to have been able to put a lot of miles in the realworld against real police radar and police laser speed traps and have been very pleased with the Whistlers. Whistler XTR-695 Review