Given that the house is already set up for Forced Hot Air, sticking with a hot air system would probably be the easiest. It also allows for the installation of A/C now or in the future if you so desired.
To retrofit the house for a hydronic system could potentially get quite invasive and costly especially if your home has a finished basement. All that piping has to get run somewhere.
A hydronic system would also need to be set up so it can be winterized to prevent freezing, so that it can safely be shut down each fall. There are no freeze up worries with a hot air system.
As far as fuels go, I would probably stick with oil. Because your usage is currently seasonal for now, any propane supplier you choose is going to charge you a premium price per gallon because you won't burn much fuel. The more you buy, the cheaper it will be. I know lot of people around the lake with seasonal homes, who are paying upwards of $6.00 per gallon for propane because they only use a few hundred gallons per year. My house is 100% propane and I use roughly 1000 gallons per year. I have to fight with the fuel company to keep my price in the $3.50 / $3.60 range
Even if you were to burn alot of propane and get a reasonable price per gallon, oil and propane are so closely priced right now that in my opinion your going to get the best bang for your buck from oil. Yes propane does burn cleaner but you get alot more BTU's out of a gallon of oil then you will a gallon of propane.
If I were in your shoes, I believe I would talk to one or more reputable HVAC contractors about a new high efficiany, oil fired, hot air system and a new oil tank.
Thats my 2 cents worth of knowledge and suggestions. Good Luck
