And welcome to CoffeeSnobs [smiley=thumbsup.gif]. When I first started out using an espresso machine, I used to try and stick with the single basket for singles, and double basket for doubles, et al. After a while though (pretty short while as a matter of fact), I found that I didnt really like shots made with the single basket as much as those from the double (I mainly drink ristrettos or dopio piccolos). Since then, I have never gone back to using the single basket.... If anyone drops around for a coffee and dont like doubles, I still use the double basket as for a double shot, but just pull the shot up short for approx 30ml. Of course its much easier if youre pulling multiple singles from a double basket..... two singles at a time.
As to what kind of double basket to use, a lot of people including quite a few of our professional members prefer to use La Marzocco (LM) baskets as they hold a bit more than the standard 14 grams, more like 18 grams+ and this leads to the production of much richer, sweeter shots. There is also the side benefit that larger baskets are more forgiving of slight dosing inconsistencies so this might also be an incentive :). Hope Ive helped a bit,