You have to compare apples to apples...
An indirect's ability to heat water depends primarily on two things (well, maybe three, but I'll get to that):
- the desired output temperature. The lower the acceptable water temperature, the more water you can get out of the thing either in a large dump or continuously.
- the amount of heat you can put into the thing. One spec sheet lists common input heat. The Weil-McCain lists it based on which boiler model you use, so you'd have to look at the boiler spec sheet to see how they line up.
the third thing is how cold your incoming water is.
So, how big of a boiler are you going to use? Then, make sure it is hooked up with a priority zone - so that when you want DHW, it shuts off the house heating zones, otherwise, your first hour rating won't be anywhere near the spec sheet since you won't be getting the full output of the boiler.
If your house doesn't need a huge boiler, then you may want to go with a bigger tank, otherwise, you'll be paying both for the larger unit and install, but having it run at less than its most efficient level.