1. Junkrat doesn't have to get that lucky. Them stats are based on per 200 dmg. Junkrat can hit a single person with 1 grenade at any range for 120/200 figure. McCree can only beat that if he's at his most effective range AND headshot them.
Now factor that Junkrat can realistically splash some of his grenades, and now you can see why his ulti, despite having the same requirements, is able to be charged much quicker.
I haven't singled them out based on who has teh better ultimate, but because while they charge up their ulti at the same rate, Junkrat is much easier to charge it.
I'd argue McCree's is better because I blow the rip tire away all the time with McCree. The only reason you could say it's better is because of the frequency of rip tires being up, at which case see above.