The increase from 22 to 33 megapixel is gradual, just as 33 to 50 is, and 50/60 to 80. So no I would not say any major improvement. For the "perceptual" resolution increase I think it's more telling to look at one side, 9um down to 7.2um than the area pixel count, and that's a 25% increase.
I think you will notice a reduction in situations where you get bad moire though even when going from 22 to 33, that's probably the largest improvement in terms of image quality. So if you experience moire issues from time to time, going for the 33 could be worthwhile. There's also the Aptus 75 which can be had at a lower price and has virtually the same image quality as Aptus-II 7, but is slower.
The Leaf series are not as popular as Phase One on the second hand market which show on the prices, so if you're into value for money Leaf is probably the better choice. On the other hand when shopping around for a good price it's best to be open-minded on which back you're getting so you can get the best deal for the moment.
The main advantage as I see it of the P25+ is that it can be more reliable. From time to time people get issues with the Aptus series, related to cold or hot weather, occasional write errors to the CF cards etc. Many use them without problems too. From my own experience with Aptus together with other user reports I've seen the most likely cause of the Aptus issues is sample variation, some backs are good, some are less so. My first Aptus 75 was dreadful if not shooting in room temperature, my last Aptus 75 was virtually flawless, even if shooting in -30C. The P25+ is instead known to be very reliable like the whole P+ series. If you shoot in tough conditions this is something you should consider, but if you're more a room temperature kind of guy then the risk with Aptus is low.
Concerning lenses, if it's good enough for 22 megapixel it's good enough for 33.
With the Aptus series, even the older, the screen is good enough for pixel peep sharpness check. This you can't do on P25+. If you find that feature valuable, Aptus is your back. If you trust your focusing skill or are not that worried about focusing precision you won't need that of course.
If I knew the previous user of the back has used it successfully in the type of shooting conditions I'm facing (my case -20C from time to time) I'd go for the Aptus, buying in the blind I'd go for the P25+. One aspect of the P25+ vs Aptus is that the Aptus looks pretty ugly on the V body, while a P25+ looks better, not as nice as a CFV back though.
