If you have PV on the roof in excess of what you can use and it's net-metered, spend the real money on mini-split heat pumps and get off the oil!
Even if you DIDN'T have the PV, mini-splits would make good short and intermediate term economic sense (unless heating oil drops back to a buck-fifty, like it's going to do any day now... after the next ice age maybe?)
In MA you can even get 7-year/0% money for ductless mini-splits through MassSave, and the operating cost during the heating season is going to be less than half the cost of heating with the oil burner. It takes a bit of careful thinking about how many ductless heads it takes, and not all houses are ideal, but most houses can manage fine with 2-3 heads and 2-3 tons of compressor. Fujitsu & Daikin both have great very high efficiency products with fully rated outputs at -20C/-4F, and the Mitsubishi "Hyper Heating" -FExx series are good down to -25C/-13F. Even without the PV, if you spend $7-7.5K (a credible number) for a 2-3 head ductless system it's usually cash-positive in year-1, if you take the 7 year 0% financing. WITH PV it's a no-brainer- get RID of that stinking environmental hazard in the basement and move onto lower carb lower-cost state of the art heat pump technology!
I can point you to a huge amount of online detail on ductless heat pump performance issues and help you figure out sizing issues, if it's of any interest. But for now lets keep it on your hot water question.
Using a hot water heat as pre-heat to the coil means you have to keep the boiler hot all summer or plumb in coil-bypass to keep heat from the hot water stream from just heating up the boiler. The heat transfer in the tankless coil works both directions, after all. It's simpler to plumb it to use the boiler as pre-heat to the tank, which won't really care if it's taking in 60F summertime water or 150F wintertime water.
If you plumbed it with the coil as pre-heat to the electric tank, you can turn off the boiler for the summer and save huge on standby loss cost and a bit on air conditioning cost.
IIRC when using the boiler's coil as pre-heat to the electric tank, MA code still prescribes a tempering valve on the output of the coil as scald-protection.
Heat pump water heaters are heavily subsidized by utilities in MA, but only if you're already heating hot water with electricity, but may make sense if you continue to heat your water with the boiler, since it's soaking up boiler standby as for about half of the hot water heating increasing the overall net efficiency, and behaves like a dehumidifier in the summer.