I've been on both sides of the table in this conversation a number of times and have come to the conclusion that the only factor that matters is sales generation.
Page ranking, etc. is all meaningless if their efforts don't result in increased sales - significantly in excess of their fees and your advertising investment.
Don't forget that driving traffic is only half of the equation. Conversion is the other - and perhaps the more important / difficult - part of the equation.
Don't pay someone just to drive traffic without a strategy to convert visitors into customers. Otherwise you're wasting your time and your money.
Ask them for specific sales results they have been able to generate for other clients. Compare the sales increases to the costs of their fees and advertising expenses. Ask for and call references of current / past clients they have generated results for.
I'm not quite sure what you mean by "an ecommerce site to compare it to another one" but, in addition to traffic stats, be sure to look at conversion percentages and sales numbers.
To your success, Frank