I am only 5'10" and we had a Cultus till like 2 weeks ago, which we had kept for last 5yrs. I have driven it and also sat on all seats in all sorts or journeys over the years. We are currently also looking to buy a small (possibly) used car and have checked out the WagonR. Personally, that's not the feeling I got. If someone says WagonR is better build, have newer technology, has more features, etc. than I would believe him/her as there is no point in avoiding reality even if one likes/dislikes it.
Usable/Relevant space, however, is another issue. I felt Cultus looks and feels more spacious. Its width also means 3 people will probably sit more comfortably. My point here is not to argue as it could be your own opinion. However, as the OP asked for advice, I feel that we should give him an honest advice and not our prejudice/personal likes/dislikes. Best is for OP to try out both and decide for himself which one is more suitable and if indeed rear leg space and width are of prime importance to him or not.
What I mentioned is a fact which remains that Cultus is both more lengthy and wider than WagonR.