You're asking a seriously broad question. Also, just an FYI … people are, in fact, going to spark a Nikon vs Canon discussion as a result.
Back to your broad question.
Nikon and Canon are both equally good. Some will say that Canon is better because they make their own sensors and such, but this doesn't make them better or worse. Nikon using third parties for their sensors and such doesn't make them bad at all.
Each lens will have an MTF chart and on that chart you can see the lens performance. Ultimately, if you're at all interested in determining which lens is best, the MTF chart is a great place to start.
My advice to you is list the lenses you think you'd be interested in going forward. For example, pick a wide from each manufacturer, a short zoom (24-70mm), a telephoto and perhaps a couple primes. You're not required to buy these, just research the lenses from each maker. Does Nikon's MTF chart (for example) show better contrast across the board on more lenses or does Canon? You'll probably find that they're going to be very similar in many respects. Why? As I said before, Nikon and Canon are both as good as one another.
Now, I shoot Nikon dSLRs and for a point and shoot I have a Canon. In my opinion and with my experience, Canon has made better point and shoots over time. I use Nikon DSLRs because I used Nikon film cameras. As I was seeing that I was growing out of my lenses I started to look into Canon. Personally, I was just comfortable with Nikon so I opted not to switch.
Again, your question is far too broad and actually very technical. If you want opinions you came to the right place.
Interesting request - but my mind just keeps trying to tell me it's the sensors and image processing that'll produce the most image difference rather than the glass..
Hopefully there'll be some nice insightful answers!
You can post process all you want, but if you take a good photo with a **** lens you can only
fix it so much post processing. It's similar to film (most around here have never used it) but you can only do so much in a darkroom to fix your error and there's little you can do when a photo is soft because the lens sucked. I'll say this until I die ... invest in glass because that is where it truly counts. Sure, the body of a DSLR matters more than it would on a SLR, but overall, if you put a vivtar 100mm on a Nikon D300 or a Canon 50D; it will still produce a less superior image than if you had shot that with a Nikon or Canon prime.