Imo the Mac which has the best performance/price right now is MBA 13" (i5, 8GB RAM, 256GB SSD). Why?
First of all, having owned an MBP 2008 and MBA 2011, I'd say that at least 256GB of storage is no-brainer. You shouldn't make do with an ext HDD and install apps on them, making them running slower than usual. If you need bootcamp or VMWare, Parallel etc. then 128GB won't cut it. About RAM, 4GB is enough for most of casual users but 8GB will ensure that the machine will stay for 3-5 years. It'll also help when you need to run virtualisation programs which demand much RAM. i7 IMO is not needed because it provides an extra 10% or so of performance, for a hefty price of $150, and you'll run into a risk of slightly compromising the battery life. If you want power, then consider an iMac or rMBP 15" instead.
Now, I'm going to compare the said MBA model ($1399) with rMBP 13" (i5, 8GB RAM, 256GB, $1699).
The only selling point of the rMBP 13" is its gorgeous retina display. Its performance is weak for such a pricey laptop. The rMBP 2012 model is weaker than the MBA 2013 according to some trusted benchmarks, and I doubt the rMBP 2013 will be significantly more powerful than the MBA. If anything, powerhouse users will need the quad core chip and dGPU of rMBP 15", not the lacklustre performance of the rMBP 13", and for casuals like me the MBA will suffice.
Back to the point about the retina display, I think it is without a doubt beautiful, but so is the display on the MBA. Whilst 1440x900 looks awful lot on paper, it's actually quite nice in reality. In addition, not so many applications right now support the retina display, so the only retina resolution you'll stare at is the desktop wallpaper, Apple-made apps and a few others. Even in a few more months, more retina-supported apps will be released, I don't think rMBP 13" can run them decently (especially games).
The MBA on the other hand, is thinner and lighter, has an amazing battery life and costs $300 less. I could have buy the 2TB WD ext HDD for $129.99 (on sale), a mouse and a nice sleeve, and still have $100 left to invest in applications. Or in my lunch for a whole month.
To sum up, I think the rMBP 13" is quite a waste of money for the performance you get. MBA 2013 and iMac/rMBP 15" is the way to go for casual and heavy users, respectively.
Nick