How do I ask for advice politely?

In practice, they're usually the same.

The "might be able to" is a bit more formal -- perhaps because it implies more hesitation on the part of the asker if this is an appropriate time and place and topic for the question. The "if you could" might be read to imply that you doubt the ability of the person being questioned to help you.

I don't think 'might be able to' is very good style, personally. Further, 'might be able to' just means 'could'. There is some difference between would and could, i.e., between 'I was wondering if you would give me some advice' and 'I was wondering if you could give me some advice'. In most situations, I would use would, as usually the person you're asking is capable of giving you advice.