The MBA uses a low voltage i5 and while it is a perfectly good processor for daily tasks: office work, browsing, etc. anything processor intensive will bog it down. It actually benchmarks close to the rMBP's base i5, though the rMBP has a much higher clock speed which matters for Final Cut Pro. It also has TurboBoost which is a big help Bumping to the 2.6 Ghz i5 is a smart buy as it benchmarks significantly higher than the MBA processors and somewhat higher than the base i5, but probably not necessary if you're just a hobbyist.
For the rest of your budget, I definitely recommend loading up on RAM if you can. It's soldered now so this will be your one chance to get it. The $1499 SKU is ideal if you can swing it. Even if you bump the processor, you're still paying $200 less with the only real difference being that you only get 256 GB of internal storage (not a big deal as Thunderbolt makes for great external storage)