...Should I opt for the 2.0 GHZ i7 (up from 1.8 i5 - £120 extra) or the 8GB RAM (up from 4BG - £80 extra)?
...out on the road I will often be working on multiple documents, researching using plenty of active tabs in Safari, as well as running iTunes, Twitter, Mail, Skype, Dropbox, Bowtie etc.
I will also be needing to do a lot of transferring of footage from CF cards via a card reader to external drives (yay for USB 3!).
Outside of work I do a bit of photo editing in Aperture, but noting too professional, just the odd polish and tweak here and there. It would be nice to have that option when I'm mobile, but I can always do it on my iMac if it will be too much heavy lifting for the Air...
We share a lot in common, including the same first name and the same profession, and I am a MBA owner and you are soon to be one. We also share a recreational use of Aperture. My workflow is therefore also similar to yours: a handful of low-CPU-intensity low-RAM-requiring programs that I like to leave open most of the time, and one high-CPU-intensity high-RAM-requiring program, Aperture, that I use infrequently. So we also share the same single bottleneck: Aperture. As far as speed and performance issues, neither option would really impact the rest of what either you or I do.
According to your requirements I would opt for the faster i7 processor. I wish I had last August. Geekbench benchmarks show it to be 10% faster. Aperture should run at full potential within 4 GB of RAM, if running with nothing else running simultaneously. Everything else you do (other than Aperture) can run fine with 4 GB or RAM, even if all running at the same time, because they are all low-RAM-intensive (not to mention low-CPU-intensive) and while there may be more VM paging (but not much more) all of that paging happens on a super-fast SSD anyway. IOW, if there are paging penalties, and they would likely be slight, they will not really slow things down because the speed of paging to an SSD is much closer to the actual speed of the RAM itself, at least compared to paging to a spinning conventional HDD.
When I run Aperture, I just shut everything else (Numbers, Omnigraffle, iTunes, Safari, etc.) down to make my 4 GB of RAM fully available, and that seems to be quite enough. But more actual CPU speed would be nice (I have the Sandy Bridge 1.7 i5). I am disenfranchised from adding more RAM (2011 model) after the fact; not sure about the 2012 model. Shutting all else down is easier than it has been in the past because everything reloads so fast from the SSD when I am through with Aperture, so it's less bother than it used to be.
So to clarify, if you take it from the position of what your world would be like with neither option, and which of those two options would improve things for your particular workflow, what you would be missing without 8 GB of RAM would be that you might have to close other programs when you want to run Aperture at its full potential (and maybe even not), but what you would be missing with the slower processor would be 10% slower processing times in Aperture (and whatever you go for in the future), and you would not have a workaround such as closing other programs to get that speed any faster.
Everything else you do would not be impacted positively either way by a faster processor or more RAM. But that may change in the future if you add other tasks you are not doing today. As far as your file transfers, I assume that would be from internal SSD to external via T-Bolt or even USB 3. Neither more RAM nor a faster processor would impact that in any way either, so nothing other than Aperture is a part of the decision between the two options. IOW, you will see little difference between a MBA with neither option, either option, or both options, until you run Aperture, and then you can maximize performance there by closing all other programs so that you don't need more than 4 GB of RAM, and you can then opt for the faster core i7 which will give you the most bang for your buck compared to having 8 GB of RAM when that program will run at the same performance level within 4 GB.
Bottom line, the faster processor will have more impact on the Aperture bottleneck than more RAM will, because you can run Aperture in 4 GB of RAM with very little speed improvement from having 8 GB available, but a faster processor will always be slightly faster in Aperture regardless, and significantly faster when you need it to be. Also, if as you say you like to keep a few different programs open otherwise, the i7 is engineered better for hard number-crunching than the i5 when stuff is happening simultaneously, assuming the software supports it.
And neither option will really impact anything else you do all that much. That said, a faster processor would have slightly more impact on all tasks than having more RAM would, generally speaking. If you want to have a zillion programs and documents open, get more RAM instead, but I think it makes more sense to manage what you do so that you do not leave a zillion programs and documents open.
Now if you said you wanted to add Final Cut to the MBA, whether 8 GB of RAM or a 10% faster processor speed would have the greater impact would need further study. Since FCP does not actually manipulate large files by moving them back and forth unless it is rendering a transition or what have you, and instead just sets pointers and displays a low-rez proxy for editing, it is probably not all that RAM-hungry like PhotoShop or Aperture would be. In that case I would still opt for faster processing except it is hard to judge which would really improve rendering speeds if your particular editing workflow contains a lot of rendering tasks (some do; some don't). I think my strategy still stands even with FCP: shut everything else down, get the faster processor, live within the 4 GB of RAM. And much of this is based on the lower VM paging penalty to the SSD than to a conventional HDD when using less RAM.
