Well, I have both. From a horse power stand point, they are identical. However, we are talking about what is useful to you. The main difference is the operating system. That is about it.
I have the M4800 that I use for Windows Development, and programming Machinery, Autocad, etc..... This is a commercial grade laptop. I could very well do the same thing on the MBP if I set it up to boot in Windows. I have also ran a Hackintosh Virtual machine in the M4800.
For me, if I had to choose one, it would be the Mac. Simply because it is a more versatile platform. I can run GNU/Linux applications out of the box(Must have X11 and GCC libraries installed) The hardware will run Linux, SCO, BSD, FreeDos, etc.... The PC cannot "legally" run MacOSX, but it will do everything else. For Music, the sound card on the Macs are better sounding.
The only thing the M4800 has on the Mac is Memory expandability, a docking station, and standard hookups without adapters. MBPs will nickle and dime you for adapters.
Lastly, I have Macs that are 10 years old now. They still run, and are still usable. PC just do not have the same track record over all. You are lucky to get 7 years out of a GOOD PC if that.
Hopefully this will help you.
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Which version of the Mac OS does the Dell run?
Officially unofficial, Maverick will run on a Dell. Have personal experience.....but know nothing about it.