S/D Vs. S/P
It depends on what you go against. If you’re talking about leveling, then S/P is a solid choice since you can drop the blind field from skill #5 on an enemy, then just auto-attack and kill them.
Leveling in general can be a bit rough on survival since thieves are super-squishy below level 80 (still squishy after that, but not as much). Here’s a quick breakdown of the weapons that may help you choose what to go with though:
S/D – Sword is a great weapon in general for anywhere that you suffer conditions. The S/D combo is also great for boon stealing or for just giving you another evade. Cloak and Dagger can allow you to easily get stealthed, but this isn’t always a big deal for sword mainhand as the stealthed ability is a great cc, but low in damage.
S/P – As mentioned, the blind field makes it great for blind tanking enemies, as long as they aren’t a boss (since blind is weak against them). Pistol whip is also a solid dps ability with a stun that helps to cc.
D/D – This is the go-to weapon set for maximum single target dps in dungeons, but you’re very squishy while using this. I would advise against this while leveling since the damage doesn’t become great until you hit 80 and it relies heavily on being behind your target. Also, this is a great weaponset for PvP, although many consider D/P to be stronger.
D/P – This suffers from the same weaknesses as D/D in leveling for the most part, but the blind field does help a bit. For dungeons, D/D is an all around better choice, but many argue this to be one of the best PvP weaponsets.
P/P – While this isn’t great for solo play, it’s probably what I would recommend for doing dungeons at low level, especially if you’re new to the game. Learning fights is huge to surviving as melee in dungeons and playing as ranged early on allows you to watch a fight without spending the whole time on your back. Pistols also do decent damage with the right build, particularly if you pick up the Ricochet talent in Trickery.
P/D – This is a very unpopular weapon combo, but I found that it’s actually not bad for soloing difficult content since you can stealth with cloak and dagger, then unleash the stealth attack from the pistol without having to be behind or close to your enemy.
Shortbow – This is the secondary weapon to almost every build and one of the biggest mistakes most thieves make is not utilizing it enough. The damage is great and throwing cluster bombs into combo fields is a great way to help out allies.
Hope that helps! Welcome to the thief class. =)
P.s. A word of advice: if you want really helpful info from a thief forum, check out GuildWars2Guru.com. There are some really cool people with lots of productive discussions, at least in thief sections.