Seras's English dub voice is OK, as I've heard much worse when it comes to annoying female voices. And the rest of the English dub cast for Hellsing is fine, especially with Crispin Freeman playing Alucard. There's nothing that I find intolerable about it at all, and its a rather good dub when compared to many other crappy ones, IMO.
Anyways, back on-topic:
I really don't mind watching an anime dubbed first as long as the voices are good. So, I'll check it out on Syfy tonight, and if I like the voices well enough, then I'll stick with watching the English dub on Ani-Mondays. However, if I don't find it to be so appealing, then I'll just go with watching the sub of the anime once I finish reading the manga (which I'm still only half-way through, ATM).