To decide, simply identify all your application "minimum" needs. If an application recommends minimum i3, then buy next model up. For example, machine with i5 processor. If application needs mimum 20 GBs HDD, then use average 40 GB HDD for that specific application. If all your applications and DATA files (movies, pictures, email, etc. etc.) need "combined" is minimum 320 Megs, then buy 500 GB HDD instead. If wondering, I always buy the next size up after raw calculations. Thus, allowing expected future growth in its CPU & HDDs as well.
Do you need portability? Personally, I love "portability" for average home user usage. re: File/Print/Email/video streaming. For average NO Games playing need, the i5 platform does great. More then enough CPU power for the typical home user. And, I put games on a proper gaming machine - like XBox 360 device - connected to my large screen LCD TV.
Do you need portability and large screen, simply buy a MacBook Pro with external 24" high res monitor. Connect via special order DVI cable. And, connect external keyboard, external mouse and external printer to USB powered hub. Simply plug the MacBook Pro laptop into DVI cable and into the USB hub, and "away you go". One has a nice large screen in a fixed postion desktop desk. Yet, still allow one to "uplug" the laptop and go full wireless - in a full "portable" mode. Like using a laptop on one's lap while watching TV commercials. BTW: I'm writting this post from my laptop - while wating for TV commerical to finsih. (giant smile).
MacAir or MacBook Pro ??? Compare each system's specs / features and compare against your application needs as well. MacAir is a very light weight and fast machine (because it uses non-moving storage). But it lacks mass amount of internal storage. MacBook Pro is more CPU power (with its larger i5 / i7 chips) and it has more internal storage. But, its heavier weight. And, likes more battery power - compared to MacAir. Thus, pros / cons with each.
In the end, only you can decide what the best tool is - for your needs / wants. If portable / fast is your thing, then MacAir it. If portable with more power / more storage is your thing, then MacBook Pro it. If fixed postion desktop (with NO portability) is your thing, then iMac it.
Hope this helps...