G'Day Tom, welcome outside.
Tough call, both makers do not produce junk.
On a pure bang for buck basis, I'd be leaning toward the Giant but I am a fanboi. Seeing as the Meridas are current models, if you can get $100-150 off the 500 then things get much closer.
Here's the thing tho'...
It's your bum that has to like the way the bike feels and your heart that has to lift whenever you look at the bike, this needs a test ride or two.
Don't forget any extra bits you'll need when working out your budget: Helmet, pump, patch kit, multitool, saddlebag and bottle/cage for a start.
One bit of advice I'll give you for free is: Once you've sorted your budget and it's time to hit the shops, tell the sales staff your budget is 10-15% less than your top dollar, it'll give you extra haggle room to talk them down and if you end up spending your whole budget the salesperson is happy that they've managed to upsell. Everybody wins.
Good luck with it, enjoy the hunt for YOUR new steed
Shaun
...whatever the road rules, self-preservation is the absolute priority for a cyclist when mixing it with motorised traffic.
London Boy 29/12/2011