I would say it would more than likely be WSTM, since that's the station already coming into the market via cable, and geographically, it's a heckuva lot closer than WPTZ. Time-Warner in Syracuse already offers WSTM's digital channels on digital cable... do they not offer them in Watertown? WSTM offers three digital channels: NBC-HD, Weather Plus, and WSTQ.
If you're asking specifically because you want the HD channel without paying for cable, keep dreaming. Neither WSTM nor WPTZ would ever put up a translator in Watertown, because they would lose money on it. Even though WSTM is seen on cable in Watertown, its sales staff can only sell commercials based on Syracuse ratings.
(I am assuming that WSTM doesn't pay for the Watertown numbers, and therefore, cannot "add" them to the Syracuse numbers. Even if they do, most Syracuse clients know they're not going to lure customers down from Watertown. Similarly, most Watertown-area retailers probably can't afford WSTM's rates, and thus, devote their TV dollars to WWNY and WWTI.)
In order for WSTM or WPTZ to make the Watertown translator profitable, they'd have to employ a devoted Watertown-area sales staff, and install the necessary equipment so that Watertown commercials can be "inserted" over the Syracuse commercials. Even if that Watertown sales staff were just one or two people, just the basic costs of the infrastructure, electricity and maintenance would make it hard to turn a profit.