I had (and still do) APIs for years, but purchased a Summit Viper before last season. Overall, I'd give Summit the edge for how it packs in/out, weight and relative quietness.
Plus 1fall gobbler said:I still like my API Grandslam. It could be a hair lighter but the comfort and stability make it worth it. It does not ever slip, creek, rock, moan or groan...absolutely rock solid and it's so comfortable I can sit from dawn 'til dusk and even take a nap in between.