A couple of very general 'rules':
The smaller the number, the more busy the A road. So the A1 is generally bigger and busier than the A1079. Although I wouldn't choose to ride on the A1079, unless I was needing to ride as fast as I can. It's not a perfect rule, but it's a clue.
The other thing is that an A road in an urban area may well be more attractive to ride than in a rural area, because it's likely to have a lower speed limit. So I'm happy on the Fulford Road, but I wouldn't be keen on riding on it once it leaves York and becomes known just as the A19.
Generally, I use A roads in town, but try to avoid them when out riding for fun, unless I really have to use one. B or unclassified roads are just nicer. The first tour I did, I rode the A1079 to Beverley, and then once over the Humber Bridge, took to the A17 (or was it the A15?) for a while into Lincoln. Madness, and I didn't know any better at the time. Much nicer to take a slightly longer route on the back roads.