Hi Taj,
I've never done any comparisons in the sound quality between those two tapes but, unless you already have a stash of Ampex/Quantegy 499, I wouldn't buy any. All Ampex tape stock produced after the mid 70's is prone to sticky shed syndrome so it's a roll of the dice whether you're wasting your money and time buying any.
There's a post by Paul Stubblebine running around here somewhere where he states that he's had 499 go sticky in 6 months.
I think if you have your heart set on using and higher output tape like 499/GP9 I'd use the new ATR Magnetics tape. It seems like many of the studios that still spin tape have adopted it as the standard now. Be aware that you'd need to have your machine biased to whatever tape you decide on to get the most out of it.
