For 99% of the population*, I recommend a laptop PLUS an external screen and keyboard/mouse. If budget is tight, aim cheaper on the display (but check reviews; Dell has some good deals). Note that you may need an adapter to use some displays with some laptops—a little extra cost.
This setup is much like an iMac (with a little more cable clutter), but you have your cake and eat it too.
Even if you don’t “need” portability, you’ll appreciate it sometimes. (Being able to work/play in any room of the house, or in bed, or at a friends’ place, or at a meeting, etc.) I know you have a laptop now, but you may find you prefer to be on OS X full time

In which case you could even sell the Lenovo for a little cash. In any case, the MacBoo Pro will likely outlast the Lenovo’s usable (and decent-performing) life, and when the Lenovo retires you’ll need another laptop. You could solve that now.
Or save money and start with just the MacBook Pro, knowing you can always add an external display later. (But if you’re going for AppleCare and an Apple display, it may need to be bought at the same time as the MacBook. Check on that. I personally skip AppleCare because my credit card doubles warranties anyway. Anyway, Apple’s displays are nice, especially with the LED backlight, but cheaper displays are out there!)
* Exception: if you’re a hard core 3D gamer, check benchmarks on the games you care about before you go with a laptop. Maybe an iMac is worth it. (BareFeats.com has a lot of comparisons with games on different Macs.)