It depends on how close N900 turns out to resemble the speculation that was leaked. If N900 comes out as leaked there is very little size difference. The length/width have slight difference due to slightly different aspect ratios of the screens but both are still 3.5".

IF the N900 is indeed a phone:

The N900 is going to come with far superior hardware with OMAP3430 and 256MB of RAM and this is the one reason I'm excited about it. The N97 is running a 434MHz ARM 11 core with 128MB of RAM.

I'm excited to see how Maemo is used on a phone.

N97 supports US 3G for AT&T and not T-mobile but it looks like the N900 will support T-mobile and not AT&T? 3G support is a must for me personally and the lack of support for AT&T 3G could be a deal breaker for me personally with the N900. Hopefully there are multiple versions as there are with their current phone.

You need to compare the software repository between S60v5 and Maemo. S60v5 is slowly getting more and more support from vendors as they support the new touch version of Symbian. Is Maemo going to get more or less support for application ports? Yet to be seen.