uhm, nope.
the only reason to choose N5 over G2 is pure Android. if OP plans to use custom ROMs anyway, there is absolutely NO reason to pick N5 over G2. It's a worse phone, with better software support from Google.
I don't think you can simplify it like that. The quality of ROMs and how many bugs they have with various handsets is important. The g2 is not supported particularly heavily (the xda scene is fairly quiet) and you run the risk of having a phone that never gets a bug free alternative ROM.
A compelling reason to choose a nexus device in the first place is to get default ROM free of bloat that removes the necessity, for many, of installing 3rd party ROMs.
The G2 is an attractive option as it ships until you scrutinise it. There is no recent apps on screen key and the update schedule is months behind the other major manufacturers (the optimus g, last year's flagship is still on 4.2!).
I think it is fair to say that the g2 has the better hardware but assuming that you can ROM your way to a good phone is not a solid decision. Neither is trusting LG to provide timely updates.
N5 or Note 3 for £70 extra!