Macs are fine provided that your never going to run any "out of the ordinary" specialist software.
If you are quite sure that you are going to browse the web, process words and spreadsheets, work with photos and graphics using the standard packages and stuff like that then the Mac will do you very well.
However, the moment you want a package to run your Film and TV props hire company, calculate your milk yields or book rooms in your guest house you will have a much greater choice on the PC.
This is as true for flight planning software as well. Jepp Flitestar and NavBox ProPlan, the two most popular packages, are not available on Mac. There may be something but you will not be able to choose either of the most popular ones.
So, there you have it, that's my opinion. If you are absolutely sure that it will always do what you want, get a Mac, but if you have any doubt and want to keep the options open....its got to be a PC (though it pains me to say so)
...and another thing. How useless is your PC really? Rather than starting again you might find that £35 worth of RAM and a reformat (or may £50 worth of new disk) are all you really need.
Try downloading AShampoo and seeing how much crap it clears out for you.
W