First of all, the two from Princess Auto have different stud holes. The $49.99 one is a 4-stud, while the $89.99 one is a 5-stud.
Second, you need to look at the load range. The $49.99 4-hole tire says it has a load range of 990lbs. I'm not sure I buy that.
The other two, (the 5-holer from Princess Auto and the Loadstar from Cerka) are both Load Range 'B', which is 780lbs.
You want to make sure the weight on each tire isn't more than the rated capacity. The calculation for your trailer would be:
1500lbs x 90% (90% of the load is on the trailer axle, 10% on the tongue) = 1350lbs / 2 tires = 675lbs.
So Yes, if your rig will never exceed 1500lbs with the boat, motor, fuel, and all accesories, then you're safe to go with the Load Range 'B' tires.
Trailer tires are such a crapshoot. I went through this last year when replacing tires on my camper trailer. They're all overpriced pieces of crap.