What option is best for 3rd floor heating?

For a quick breakdown:
-2 family home with 2 separate boilers (steam heat)
-2nd and 3rd floor are one unit
-3rd floor was converted from attic into a 425sqft master bedroom. Insulation is ok. Open layout. 
-when they converted the third floor there was no direct heat source put in. 
-I live in this unit currently but eventually do plan to rent it out within the next year or two. 

So basically, I need advice on what my best option for heating would be. I've had 4 considerations so far: running a pipe for another radiator, running a gas line for a vented wall furnace, installing a mini split, or putting in electric baseboards. 

I had 2 different people give me quotes on running a pipe for a radiator from the basement to the third floor and it's looking like it'll be $ which is far more than I expected. 

For those experts on here who know all the pros and cons of each heating type, what option sounds like it'll be best. Ideally I want to balance cost, effectiveness, longevity, and ease of maintenance as this will be a rental property. Feel free to throw out something else if I'm missing any other solid options.

Thanks!