I just went through the same dilema. From what I gather Toplac is an enamel and Brightside is a polyurethane so Toplac will give a deeper gloss, Brightside will be more durable.
I went for Brightside in the end, but more for the colour than the paint. Brightside is still very glossy, and I'm sure Toplac is very durable.
About 3 quid a tin for 750ml.
Toplac is a silicon enamel and has a better UV resistance than Brightside.
Brightside is polyurethane enamel and is very easy to apply.
If you go to the International website for yachtpaint here:
Both are good. Although I would always use Toplac, good durability and excellent finish. I am told by my local supplier that Internatuional are going to drop Toplac, although as ever with International it will probably reappear under a new product name. I hope it does, Toplac is the first decent paint I have ever used from International!
My topsides are painted with Toplac, which I find lasts 3-5 years depending on the abuse level. I can't speak for brightside, but colour retention is good: I retouched some 2-year old stuff this spring and you can't see the join.