Well, whatever you choose I hope you have a good look at the small print scruggs. At least Dell will take it back, albeit with significant displeasure on your side just as Binos faced regarding an Apple repair.
Most Mac users update machinery at a much slower rate than others. The software and OS seem much more able to keep daily use fluid on an old machine whatever the benchmarks say.
Once on the dark side though getting a new Mac is one way the evil ones keep us in the fold. Start a new one and the screen asks if you have an old mac. If so please connect by firewire and restart it. Click one button and it transfers the lot - all data, settings, preferences, software, addresses, pics, music, vids, dodgy bookmarks Etc in the original folder topography you created over the years.
A very sneaky way of getting us 'normal' folks to upgrade more often than non power users/ enthusiasts usually do.
Like changing banks it's traditionally all such a pfaff that you put it off.
Anyway, what is it that you do that suggests the desktop/tower to you? Fastest video card and most memory? Games,flight simming, video rendering, large scale stills manipulation or music production are the only areas I can think of where you'd notice the extra speed. Or you need a third party monitor with guaranteed colour gamut and measurement? With the rate of change in memory architecture, connections and slots I don't feel they're nearly as future proof as we used to consider them.
Oh, and slipping back to John Lewis and tellies again. They don't give any of this HD Ready, Almost Ready, Might be Ready if you are partially sighted tosh. It's 1080p or it ain't in the list.
It's OK - I'll thread ban myself
Rob