DivineAngel

Grand soul gems are the highest, it's possible to find them in loot (including in the plains of oblivion) but you can also buy empty grand soul gems from alchemists in the mages guild for a hefty price.
As for blood potions the only one is the one you find in Vincent Valtiere`s desk in the dark brotherhood sanctuary, at least to my knowledge. Which form you prefer depends on what you like to do really. Personally I think becoming a vampire is a novelty that wears off over time. When you become a fully vampire there are also a few restrictions that are placed upon you though you do gain new powers over the course of becoming a full vampire. Firstly, you will not be able to withstand the sun, it will kill you rather quickly making a lot of daytime quests difficult, if not damn impossible to do. Also, People will begin to loath you for who you are making social interactions impossible and that goes for bartering with merchants too. You can get round this one by using the vampires seduction power (a charm spell) that will help especially when you need to barter. If you want to stay as a vampire and yet complete any daytime quests you need to drink blood. Obviously this will diminish your full vampiric powers and leave you at the first stage of being a vampire (there are about 4 I think with stage 4 turning you into a full vampire) but at the first level you will not be harmed by the sun, and when you have done all that you needed to do during the day, you can then slip back into becoming a full vampire again (by not drinking blood).