I'd be scared of screw in...if by some weird chance the tap breaks, or cuts crooked, you could need a block...I would go for brass if given a choice- they might not last any longer(dunno?), but I would think the softer material would knock in easier- less risk of hurting the iron...
thought: IF someone were to tap out their block for screw in freeze plugs, I would say the block stresses would be bad enough to weaken it, and possibly ruin their 'torque plate honed' round cylinders from the stress of a tapered pipe plug being screwed in- Or am I way off and screw in are actually straight thread/o-ring types??? never actually seen one out of a block.
edit- btw- often replacing these 'in the car' is nearly impossible- if you cant get to them you might have to resort to the (yuck) rubber type with expanding screw/nut...they work, but Ive seen some not last but a few years...Dad had a challenger w/318 that garage put these in- cheep fix, but started leaking after couple years- easy to swap for sure though.
<font size=-1>[ This Message was edited by: ford4v429 on 4/18/06 9:58am ]</font>