Well the processor is slower on the mini, but that may not be an issue if you don't do anything intensive. You also get more space on the iMac since 640GB seems to be the standard now.
There are a few sacrifices, but yeah, if you get a MacBook/MacBookPro/Mac Pro then you are in a better position not to have to spend more money on a big monitor. You are also in a better position to build a gaming rig and use a KVM switch to switch between them (assuming you can somehow convert the video port).
I'd go with the iMac personally, but at minimum the $1799 model. Yeah, a couple extra hundred but it would be worth it. If possible I'd even squeeze a bit more money into it and get the best video card possible to future proof it. The 9400m is a crappy card now (barely acceptable), imagine what it will be like in a year or two (most likely on the same level as a Intel 950GMA is today).
Over 24 months of ownership that comes out to be roughly $80-$85 a month. If you keep it for 3 years, that's around $60-$65 per month.
In either case, if you plan to keep it for a while its best to just get the best spec you can get now since you can't really upgrade the video card yourself on an iMac and even though today the 9400m may be acceptable for your needs it may not be the case in a year or two.
I wouldn't count on selling Apple products without taking a big hit either. With Apple becoming more popular I have found it is harder and harder to get a lot of salvage value. It used to be the case that if you bought a $1500 Apple computer today you can sell it the following year for maybe $1200 or $1300 -- barely taking a hit -- especially if it was in good condition.
Now with everyone and their momma going to Apple, you'd be lucky to get 60%-65% of the value after a year. I had to sell my less than year old MacBook Air with SSD for about half of what it was "worth" when it was new, and I got lucky too because there were other cheaper sellers too.