As a rule of thumb, entire males grow better than wethers except when they get hormonal. So it's a juggling act. If they'll finish off grass before the tupping season starts, then for sure entires will grow faster and probably better (more muscley, less fat) than wethers. You can cake entires harder than wethers; wethers will slab on the fat more readily than entires.
If they need cake to finish before tupping starts, then that's a judgement call and you need a crystal ball to predict when the fat prices will be best! We find the prices to be in general best before mid-July, then they drop a little as everyone's lambs come to the mart, and stay flattish till the new year, when they often - but not always - rise again.
And yes, it's to do with the season, the slowdown in growth. It's maybe more marked if they've ladies nearby, I'm not sure, but I don't think two fields' seperation will make much difference, no.