The main difference is how they were designed and rated. They both do both.
The deciding factor should be how you plan on using the unit and what features are most important. Then the final thing to consider is which unit fits your needs without compromises. <See my tag line below>
Use the biggest hammer you like, pounding a square peg into a round hole does not equal a proper fit.
Well I have a 1 1/2 storey heated with a medium efficiency 125,000 BTUH input gas boiler, and a separate medium efficiency gas water heater. The house is heated with cast iron radiators fed with large gravity feed pipes. Only two people living in house with one bathtub. Would like to migrate to a single source high efficiency mod/con heating plant.
Originally thinking mod/con boiler with instant DHW was way to go, but then started reading about possibility of a potable mod/con water heater for space heating. Seems that the potable water heater would be slightly more expensive to operate due to larger volume of water being maintained hot in the hot water heater. What other issues are there to consider ?
Of course the combi units will modulate (turn down) so the oversize problem is minimized AND the con boiler just loves those big rads with no chance of the problem of low return temps SO twith outdoor reset you may have a really sound approach under consideration
You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!