Unless you're playing relatively new VR games or doing high-end video/photo editing, they're both kinda overkill.
For the high-end games and video editing, the P4's still top of the heap. The PM (Centrino) wins out over the P4 on Excel (spreadsheet) programs, and a few other office apps. This is due, I'm told, to the huge internal cache. It also uses less current than the P4, which might help battery life a little.
The high-end games and video-editing demand a fast processor. But, for most other uses, you won't even notice the speed difference between a 1.4 GHZ processor and a 2.8 GHZ processor. However, you WILL notice a big difference in their price tags!
The Centrino was developed for laptops, and for low current usage. However, in the current PC "world" a 1.4 Ghz processor is 'SLOW'. New desktops are being shipped with 3.2 and 3.4 Ghz processors.
I've seen some reports that a faster centrino is in the works (or may even be out), so if the battery life is important, look into those.
The only other comment I'll make is, I do part-time computer support and I do not recommend Dell consumer products to my clients.