Hey, I have a 1700 Sq Ft House with an old Utica 101,000 BTU Oil fired (Becket Burner) Boiler. I have been toying with the idea of replacing this unit with a newer one, as we plan on doing some upgrades in the house, and increasing the square footage to 2250 sq ft. which includes a finished basement. What is the most efficiant burner\ boiler on the market that I can install to save some oil? The heating system in the house is Hot Water w\ Slant Finn Radiators. Too, the house always seems to take a long time to come up to temperature.

You are not going to save oil by increasing the size of your house.
My recommendation is to go to Google and search for oil boiler manufactures.
The reason is that new products are getting to the market faster that anyone can keep up with them. This way you will also have the latest info available.

I also recommend getting two estimates and at that time you will get there recommendations for a boiler. The reason I say this is that you want the company you are buying from to have experience with the boiler product you are getting.
I have seen to many folks get stuck with a piece of equipment that no one can or wants to fix and naturally you want to keep the new boiler warranty so also check with the manufacturer to make sure the company you pick has attended one of there high efficiency courses.
The new high efficiency boilers (save money) but only when they run correctly and you will have to depend upon your HVAC contractor for that.

The tools and settings on the new high efficiency boiler make it more difficult to check the performance after the install and the units are definitely not user (homeowner) friendly for repair. The settings are so precise to squeeze the BTU's out of the oil but you are paying for high efficiency so you should have it serviced by a pro.

As far as system recovery baseboard heat is not fast. I usually suggest to set the thermostat and leave it alone or get a thermostat that has a intelligent recovery ramp built in so that when you program the heat to go down at a certin time the thermostat will estimate the heat loss and start bring the temperature back up in time to meet your heat requirenments for the next period. Honeywell has this on some of there digital thermostats and it works great but remember baseboard heat is slow recovery.