Let's assume the OUTPUT is 115,000BTU. (If the input is 115,000BTU, then the output would be less than 115,000.)
HB Smith which is made in Westfield, MA, is apparently a good brand. They're in the Smith-Mills line of large, cast iron boilers. They've been around for over 100 yrs. (Go to The Mestek Companies. Look for 'Smith Cast Iron Boilers'. There's a link for them.) Most of the mfgrs. apparently use the same foundry for the boiler itself but the differences usually lie in the size of the boiler and the piping that's used. If your current boiler is adequet, and you're not planning on expansion in the house, then go with the same size boiler. If, on the other hand, your current boiler is under-sized or you plan on adding to your house, you'll need to re-size. As for efficiency, find out what the current size of your nozzle is (in terms of gallons/hour), and see how it rates with what you want to buy. Also, it's suggested you get some Heat Loss Calculations from the company that you pick to be sure you're not over-sized or under-sized. They should do it at no cost. Maybe you can offer to do the legwork for the calculation for them by doing things like measuring the amount of baseboard heaters you have, room sizes, bathrooms, etc., etc.. You have to be sure that the baseboards you measure are actually heaters (you'll see fins on them). If you can take off the front plate of your baseboards, you may find out who mfgrs. the heater in all the rooms in the house. This will help in determining your heat loss.
If you want to improve effeciency, you might want to consider separating the HWH from the Heating system. In your current set-up, I'm assuming that Hot Water and Heat are coming from the same boiler. You see, in the Summer, the boiler has to fire up every time you want hot water to wash your hands. This could be costly. Also, if you have natural gas for your kitchen stove, you can look into hooking your HWH to natural gas. That way, you only fire up the boiler for Winter heat. (If you did this, it's very likely that you can down-size for the boiler. Also, it may cost more up front but in the long run, for sure you'll save some $$$.) These couple of ideas would make the most sense if you plan on staying / keeping the house for a fairly long time. If you plan on selling soon (and that's a relative stmt), your investment in a HWH/H system will be a factor in deciding which way to go.
My best to ya and hope this helps.
PS: My 'best man' manages a Boiler Service. It sounds impressive coming from me but I have to give 'The Governor' the credit!