Well, let me say right out that I have a three month old Great Plains .54 caliber flintlock with the slow twist (ball) barrel, and I love it.
They're both Lyman guns with the same lock and same barrel quality. So, it's pretty much a matter of what you want as far as style, barrel twist, length, and features. The GP has double set triggers, which I like (some don't). The GP barrel is 32", the Deerstalker is 24". The GP can be had in 1 in 60" twist, or 1 in 32" twist. The Deerstalker is 1 in 48".
I'm sure you have shouldered your brother's Deerstalker. How did it feel to you? Have you shouldered a GP? One or the other might feel better to you.
For me, the GP was worth the price.