I have tried both and there isn't much difference between Vista Premium and Vista Ultimate.
With Vista Home Premium you can back up your work to a HD like you can do with Leopard.
I'm pretty sure that you are also able to use Remote desktop with Vista Home Premium although it maybe more advanced in Vista Ultimate.
http://www.microsoft.com/windows/products/windowsvista/editions/choose.mspx
I am aware of stuff like Garageband as well but at the same time when i bought a Mac i paid £65 to have a paint program.
There are a few other differences as well the Dell has a HDMI port an ExpressCard 54 mm slot and also 8-in-1 removable memory card reader.
Like i said i think that they are both good laptops but i think that the MB is maybe more aimed the mid market and maybe higher end market.