Future Gohan would win. He appeared to have a decent degree of mastery over his SSJ form and he was capable of holding his own against both Android 17 and 18. Super Saiyan Vegeta who utterly decimated Android 19 was effortlessly picked apart by Android 18 who wasn't even serious. Future Gohan however, handicapped due to having only one arm, fought well against both androids whereas Vegeta wouldn't stand a chance, especially since he was killed by them in the same future. Android 19 would lose. Also, 19 shouldn't be more powerful than an SSJ anyway given that Goku fought well against him until his performance was impaired by his heart disease.