The funny thing about memory cards is that cost to size isn't always linear. For example, you might buy a 4GB card for $50, but the 8GB might only be $75 or it might be $150. That's just an example, the threshold for where the big price shift happens changes as technology and capacity improve. So, in terms of price, it will depend on the capacities you want.

For general pros and cons:

Single Card Pros

• Only one card to carry around, no swapping required
• Likely to be a cheaper option

Single Card Cons

• All eggs in one basket, so if toasted, you lose it all
• If you run out of space, you're stuck

Multiple Card Pros

• Reduces the risk of losing everything if one card is bad
• Allows for seperation of work
• If you can afford it, it can give you a lot more capacity

Multiple Card Cons

• Can be more expensive to get to the same size
• Easier to lose them (since they're not in your camera)
• Swapping

To be honest, the eggs in one basket scenario worries me bit, so I usually carry more than one card. Also, despite using 8GB cards, I have run out of space at times and have been grateful to have my spares (I'm paranoid, I have 3 8GB and 1 4GB cards in my bag).