Alterations at RK Bridal and Designer Loft

I found a dress I really liked at RK Bridal from San Patrick. However, I'm also going to the San Patrick trunk show at Designer Loft on Saturday. Let's say I have to make a decision about where to buy the dress, which store charges more for alterations? And which does a better job?

For instance, let's say RK charges me $1300 and Designer Loft (with the trunk store discount) charges $1400, it would seem to make sense to go back to RK. But it would make sense to buy it at Designer Loft if their alterations are cheaper right?

Plus, I heard some girls say that RK Bridal isn't great for alterations. Why so? What's the biggest problem? How is Designer Loft in that area??