StrikerS is twice the length of A's, and it seems like the writer creamed his pants when he found out he got to write 26 episodes this time - but he wasn't very good at it, and the pacing is pretty bad, by which I mean the main characters don't start fighting other mages till about halfway through the series. As a result, if you don't like the new characters, training episodes, and character development (which eats much of the first half of the series), and are bored by the main characters fighting massive forces of magic robots rather than real mages, then the first ten episodes are gonna be torture for you; I liked the new characters, myself, so it wasn't too bad for me.
That said, when it finally does get around to stuff happening, it's pretty epic, with over a dozen bad-guy mages and EVIL PLANS on a whole different scale from before, though some of the fights aren't as good as they could be.
Also, flying (which everyone could do in the first two seasons) is now suddenly an expert skill available only to really good mages, so none of the good-guy characters can fly unassisted (although some of the bad guys can), and the new characters generally have a lot less destructive power than was thrown around in A's. It's handled well, though, and Nanoha and crew still get almost their fair share of good fights.
Generally, I'd say that A's is a M&M, small but completely delicious, while StrikerS is a Tootsie Roll Pop, tasty but you've got to keep at it for a while to get to the delicious chocolate center.