As with most things, this is a subject that gets more involved the more you get, err, involved. I won't begin to pretend that I'm an expert on this subject, but I've worked on projects with friends who are, and they've been installing very expensive LED lighting systems into large hoses, and in a nutshell, it's a bit of a science.
As I say, I don't know much, but I'll tell you what I know...........
12v MR16 gives a nicer light than 240V GU10. There's not a huge amount in it, but it's a difference all the same. GU10 used to have a habit of tripping breakers when they "blew". They would also kill cheap dimmers sometimes when they blew. They now tend to stretch the filament over a "pinch" in the lamp glass which stops them shorting when they blow. Lamp life can be variable with both. In the case of 12v lamps it's dependant on the quality of the transformer and whether it "soft-starts" the lamp. GU10's are hard started all the time, which is when they are most likely to blow.
If you fit a good quality dimmer (not the trash you pick up at B&Q) like Lutron, you will enjoy the benefits of virtually never having to replace a lamp, and you will be saving money because you aren't running the lamps at 100%.
I have a kitchen with 22 12v halogen lamps and I've replaced 2 in the last 3 years. They seem to last indefinitely.
When you run the lamps at 90% you'd be hard pressed to see the difference in light level.
Okay, this is maturing technology and things are changing all the time, so in a few months the situation could be completely different.
Lights have a colour temperature, basically how warm or cool the colour is. Red at one end of the scale and blue at the other. A cool light will be blueish, and in the case of fluorescent lamps we'd call them daylight tubes.
Incandescent lamps (like 12v or 240 volt halogens) are often described as having a warmer colour, especially when run at less than 100%. in our homes we generally prefer a warmer colour of light.
Some science..... but don't shoot me if I get a detail wrong here..
LEDs produce white light using a trick. They are basically blue LEDs and a yellow phosphor which when excited by the LED produces a wider range of colours. But the thing is that when you look at the light spectrum produced by an LED light, it's quite "lumpy" when compared to a regular incandescent lamp.
in a nutshell, a cheap LED lamp may have the right colour temperature, but the light it produces looks weird.
This is because it's producing more or less of certain colours and our eyes can sense this.
It's known as the colour rendering index (CRI). the index goes from 0-100 with 100 being a perfect spectrum of light. The higher the number the better.
To get a higher CRI you have to spend more money. Also, the higher the CRI the less efficient the lamp becomes. So the more expensive lamps with good CRI are actually slightly dimmer. So to produce the same amount of light as regular 50W halogens, you need more of them.
I believe that no house should be without dimmers in at least the main living spaces. Having the "big lights" on full whack all the time is not nice. Others may feel differently, but that's just my view.
When you dim Incandescent lamps (Halogen, good old bulbs etc) they change colour, becoming redder as they dim. We as humans are used to this and we like it. It gives a room a warmer glow.
LEDs do not change colour as they dim. It's not a criticism, just an observation.
Some LED lamps/fittings are not dimmable, because they're cheap.
Don't forget that if you install or alter LOW VOLTAGE light fittings (not the lamps) and they're not "pre-made" sets, then the work will need signing off by a registered sparks or building control.
It's a bit of a minefield, innit?
Dunno if my ramblings are of any help, or even understandable, but hopefully they are of help.
Please feel to set me straight if I've made an error.