Between the two chargers, I would use the Battery Tender Jr.
As others have suggested, get a pigtail with SAE two pin connector (should cost less than $8), and put it on the battery.
Charge the battery overnight. Then put it back in the bike with the pigtail installed. I use one of those plastic wire bundle / cable ties to fix the end of the pigtail to the frame where it is easy to reach.
Then you can keep the BTJr. on the bike until riding season, and it should be very good to go at that time.