I'll start of by saying that I live in the UK. Sorry for not putting this in the original post.
I think that by MG249Q you mean MG279Q. This monitor is £50 more than the MG278Q, which I can afford, but it is listed as having 4 ms of latency, compared to 1 ms for the MG278Q. As I'm not too worried about viewing angles or colour reproduction but do care about latency, the MG278Q seems like a better option. The same applies to the XF270HU.
The XG270HU is £550. I can only assume that it is cheaper in other countries, but here it costs more than the £410 MG278Q.
I like the sound of LightBoost, and it is useful to know that it works on Linux, but the XL2730Z is £600, which is out of my budget.
All prices are from amazon.co.uk. If you know of a better place to buy from in the UK, I would very much like to hear about it. Likewise, any further advice would be appreciated - I am a long way from expert in these things.
Edit: I'll also address Miggy's post. I'd love to support Flan in the way suggested, but I believe that to buy from Newegg, your affiliate website, I would have to pay for international shipping from the US, which I suspect would not be cheap. Sorry.