Thank you everyone for your comments! I think I've come to the conclusion that I need to get a lot better at Cantonese before I attempt Mandarin. :)

I am studying both, but I wouldn't suggest beginning both at the same time. Don't listen to people who say that Cantonese is useless. Everyone is different. Not everybody who speaks Cantonese also speaks Mandarin. Even if the people who you want to speak to speak Mandarin, if they prefer to speak Cantonese then you should study Cantonese. My wife speaks Mandarin, but my in-laws are from Vietnam and they don't speak Mandarin. They speak Vietnamese, too, but they prefer Cantonese and whenever they get together with their family they always use Cantonese. Being at a family gathering and not knowing what other people are saying means you don't really feel like one of the family. You did not say what your motivation for learning was, but I doubt it's just for making money. If so, of course learning Mandarin would be better in most cases.

I speak both fluently as my family speaks cantonese and I have a alot of friends that speak Mandarin.

In my opinion though, I would learn Cantonese first as cantonese may be a bit harder to pronounce, you may find Mandarin a bit easier after learning it.

For what I have experience, cantonese speakers tend to learn mandarin better instead of the other way. Mandarin speakers usually have an accent or find it hard to pronounce accurately when they learn cantonese.

I think it will not create confusion to learn both Cantonese and Mandarin at the same time, but if you learn Chinese on practical purpose(commercial, for example), Madarin is enough. In the time 20 or 30 years ago, for westerners the commercial function of Cantonese is more important by far than Mandarin, but doday things are the contrary, for almost all Cantonese traders understand Mandarin while the opposite is not the case.

Actually you'll find it rather easy to learn the other of Cantonese and Mandarin after you have learnt one quite well. Cantonese speakers cannot communicate with Mandarin speakers in respective dialects, but the primary barrier is only the pronunciation or the phonologies. When a Mandarin speaker is listening to a Cantonese speaker, he/she just cannot associate each syllable/character with its equivalent in Mandarin so that he/she does not understand the speaker. If the speaker writes down what he/she was saying, the listener will catch it in no time. So if you do not have difficulty in grasping the phonology of both of the two dialects, it's okay to learn them simultaneously.

I am in the same boat as you, my girlfriend is Cantonese so I have been studying Cantonese for half a year now. It's useful when I went to Hong Kong and Macau, I was able to get by with basic phrases and such and its very useful since majority of the Chinatowns around me speaks Cantonese so I am able to practice with native speakers.

Accidently during my travels, when I went mainland, since I had studied Cantonese I was able to pick up Mandarin versions of Cantonese words and thus informally started learning Mandarin and was able to communicate there as well. For example:

Cantonese: yiu m yiu = want not want

Mandarin: yao bu yao = want not want

Cantonese: chi sor hai bin do ah? = Where is the bathroom?

Mandarin: chiswore zai na li ah? = Where is the bathroom?

Ever since coming back I have started picking up bits and pieces of Mandarin as I go along learning Cantonese but I am still putting my main focus into learning Cantonese. But so far everything seems to be going smoothly.