I just had the same decision to make and ultimately chose the Yamaha. But it was somewhat a personal choice that may not relate to what you want, and one that I still can't say with any absolute certainty was the best for my needs. Though, so far, I'm very satisfied with my decision. If you're completely happy with your current Marantz, you're likely only going to be even more pleased with the 7008 and it'll be less of an adjustment. Me, I was looking for an entirely new experience. Coming from over ten years of using Denon's, the Marantz may have seemed too much like the safe bet at the time - they're at least very similar in terms of features.
There is the perception of better build quality, reinforced by reports that Yamaha appears to be the most reliable of all the major brands. Whether they're really built any better, I never found a straight, well qualified answer, with more "I's in favor than against, whether it means a hill of beans to sound quality or might have an impact on product longevity is another matter. I've also heard that the 3030 runs cooler than the 7008. As AVRs go, the 3030 is about the coolest (or more accurately put: least warm) AVR I've used, putting out less heat with 7 channels driven than our previous Denon 3808 put out just being used as a pre-pro.
So far I don't think YPAO is as refined as Audyssey XT, much less compared to XT32. But with Yamaha you have more manual control via PEQ, so ultimately they should work out as well, if you're feeling adventurous enough to learn and or invest in a more thorough analysis of your rooms acoustics than just trusting the accuracty of an algorithm - which even XT32 isn't infallible, hence the benefit of additional manual tweaking via Audyssey Pro. But I think more people might be satisfied with XT32 than YPAO. I suspect both solutions may prove more evenly matched in fully treated rooms or rooms needing less correction.
It's been said, it's best to choose the AVR with the features you want most. I'm not sure how well these two stack up in that sense, as most of the features in AVRs these days are superfluous to my needs - I mostly use an AVR for audio reproduction from BDs, DVDs, and occasionally CDs, no internet music service, no MP3 files, no web surfing. I'm pretty traditional in terms of the features I want - performance is what I'm most after. That said, Yamaha's DSPs might be worth a look or listen, especially for music. I generally have never liked DSPs, but I'm not closed-minded toward them like some purists, and so far I've been very, if not extremely impressed with the effort that Yamaha seems to have put into theirs, not just their basic aural signature or design, but their surprisingly flexible implementation. Yamaha's DSPs are on another level from Denon, which I believe Marantz is close to in that regard or worse, from what I've been told.
If movies are all you're interested in, and you want an easy, auto-solution for EQ, the 7008 is probably going to be the better fit. If you're also really into music, I'd definitely check out Yamaha's DSPs though - demoing in a store and/or reading the manual online. And if you are feeling adventurous, as I was, in learning exactly what makes your HT tick, rather than relying on an auto solution, hoping it fixes everything, the Yamaha is better suited toward that as well. You've still got YPAO to fall back on, if that proves to be too much like work - which, as someone completely unfamiliar with manual EQ I'll admit it still seems semi-daunting to me, especially REW, though Omnimic2 and another is supposed to make it a lot easier. You might also check your current XT curve to see if it's doing anything below 31Hz, as that's the lowest the Yamaha can adjust EQ manually or automatically. If you need correction below 32Hz, you'd need an external low frequency filter from Behringer, Omnimode or someone to reach those.