The Big Agnes pad sleeve is pretty awesome but those 2 bags are not even close to same quality. Not a dig on BA bags but thats apples and oranges between those 2. The Sonic is double the price typically. I would jump on a deal on that Sonic bag if you can get one for not much more than the BA bag.
I would be the 1st in line if I could get that kind of deal on them. The down bag should give you a lifetime of service if properly cared for. The Sonic bag weighs less than half of what the BA bag does so it would be a lot more versatile if you ever wanted to pack in with it. Truck camping doesn't really matter but you would at least have a lighter option if you wanted. That my 2cents at least.
Well just based on the specs for a long bag in the Sonic the BA bag has 11" more at the shoulders. I own an older Hog Park bag from BA and it has same specs as the Elk Park. Its a very spacious bag as you know. Dont know if there is a bigger one out there. Sounds like you are a big dude so maybe that BA bag will be more comfortable for you. If you dont plan to backpack with it you will be just fine. On the flip side though if you can get a stellar deal on that Sonic you cant get hurt if you buy it have to resell it because it doesnt fit.