MBP Haswell - what GPU ?

#1 That is really all personal preference and what attributes one values most. All people can tell you is where their respective preferences lie.
Both can handle the work load equally well. Matte screen is something I wouldn't want to do with out the retina is also not as bright. Retina is thinner and has two TB ports, better cooling.

#2 That remains to be seen. The most likely is a 750M which really is just a 650M with a higher clock. In Windows you can overclock a 650M yourself and end up with one bought today at about the same clock speeds or close enough to not matter.
It is not entirely impossible that they add a 760M which would be a bit more of an upgrade but also not game changing.
There is really no significant upgrade possible until 20nm GPUs show up in 2014. Everything today is just rebranded chips that are now built on a more mature process and overclock (run cooler) a little better than the first batch. It is more like a new stepping rather than a new GPU.

If they remove the dGPU as some like me would prefer and only put a HD 5200 in the performance would even be lower than now (for gaming anyways). That is rather unlikely IMHO. More likely there will be one entry version with this configuration.

 

The matte used to be the brightest screen with 350-400 nits.
The retina is below 300.

 

Difficult to do prediction, an update could occur by the end of the month, but if it doesn't, I'm not sure it will happen before the end of the summer.

1) Apart from what was already said, I can say that the Retina screen offers higher resolutions for gaming, like 1920x1200. Some games will run smoothly at that resolution, I myself use it.

2) In terms of graphic card, I'm not sure we'll see anything and if we do it will be a small bump in terms of performance so nothing dramatic this year I believe. So I wouldn't hold my breath and wait for it.