PSU for SFF Rig

[TheCaptain53]

Today I ordered the parts for my new rig which is going to be a small form factor build in the Silverstone SG13, so it'll be pretty cramped. The only part I'm missing is the power supply. I've got 3 ways to go about this:

1. Buy an EVGA 650 G2 (£80) as a slight upgrade to my main rig and put my 650 GS in the little rig. It is an ATX PSU and the cables would be pretty long for the case, but it's a relatively short PSU considering it's ATX and it's a damn good one at that.
2. Buy an SFX PSU from the likes of Silverstone or bequiet. Whilst the PSU is much shorter and the cables would be a more appropriate length, there's not many PSUs available, and they're pretty expensive given their size. I can't seem to find many reviews of these units either, like the Silverstone SST-ST45SF (450W 80+ Bronze). 
3. Buy a larger, better PSU for my main rig, such as the Corsair RM850i. This would be more expensive and come with the same drawbacks as 1., so the only advantage is that I can accomodate more power hungry setups in the future. I'm not leaning towards this one so much.

Thoughts? I would put the 650 GS in there, but the cables may be too long, so I'm not entirely sure.