Can only speak for Dolce Gusto and Nespresso, but depending on what type of coffee you are into then each has its' benefits. If you like espresso coffee, and aren't too bothered about longer, coffee shop style coffees then Nespresso is a good option. If you canvass opinion from the forum massive I believe there is a number of users that really like the coffee from their Nespresso machines. The patent on the Nespresso capsules has just finished, this will no doubt mean there will be a number of other coffee makers producing coffee to use in their machines.
As for Dolce Gusto, the machine can still make espresso coffees but the range is limited. The coffees are more akin to what you'd get in a high street coffee shop, you can get lattes, mochas, hot chocolates, ice coffee etc... but can only use Dolce Gusto capsules. This is the same (if I remember correctly) for all machine makers, you have to buy their capsules. Dolce Gusto costs between 24p and 48p a cup, I forget the Nespresso cost.
As far as difference between models there is very little, mostly it is aesthetics but they will all try to sell it on some magical wisdom inside the machine.
If you are interested in Nespresso John Lewis allow you to go sample coffee, as do a number of other stores, they may even do Dolce Gusto as well.