I'm personally a Collinite fan & have talked to the local distributor (who worked in their factor for 9 years) as well as current factory employees about their waxes. I personally haven't used 915 (I've used 845 and 476 on cars + 925 + 885 on boats --- and been quite happy with them on, but I've got a sliver metalic car, so nothing will produce a shine like a dark color car). The 915 is the paste wax that should last longer than 845, but should have more shine than their longest lasting 476. The other thing I like about Collinite is their entire family of waxes is quite reasonably priced in comparison to some other waxes.
I personally haven't used Souveran, but I do know it tends to get high ratings from forum members. I don't know much about nattys blue.
With winter coming, I'd suggest putting 915 on for the winter coat. Allegedly Collinite waxes are most durable when applied directly to paint (as compared to being used as a topper).
Come spring, you may want to play with this if you've got time (and money): Apply polish or paint cleaner, apply CG BlackFire, cure 1hr, apply Collinite 845 or 915, cure 24hr, apply CG V7. I'm currently using CG BF + 845 + V7 on my light silver car, and it shines.
On cool days --- with the cap still on the 845 bottle, run it under hot water from faucet, or put it in a bucket with hot water for 1-2 minutes. Then shake (warm & cool days). Do not add water to the bottle.