get measured at a bridal salon or at an alterations tailor? What is the difference?

Depends on who does your alterations. I'd personally work with the same store I got my dress from, just because they are use to working on the styles & such. As far as how long to wait to get it altered, that's something you need to ask. David's Bridal did mine & had it ready within 3 weeks. Some places also have a rush service of a couple days if you pay extra.

What if?

Clearly you are not satisfied of your weight and your interested in losing a few pounds.

Have you set a time period to lose 10 - 15 pounds? If not, then you need to do so.

When done you can move towards getting measured for a perfect fit which may not need any alterations.

Your measurements will be based on your body physique and your interested fit ( snug, loose, comfortable, comfortably loose ) and the tailor measuring. Different tailors use different measrement styles, though there is a standard measurement for females.

You should use the store where you got the dress. Just becaue you have to order the dress does not mean that you can not get it altered in the future. Just try it on one month before the wedding and if it doesn't fit have it altered.