If the main tube is the same size,the only thing that I can imagine is the field of view set on 5X and 6X.
BTW . . . a 30mm maintube will give you more FOV than a 1" main tube scope at the same power setting. --- SAWMAN
From similarly brands, the only 'limit' for 1-6s is they weigh a little more (and cost more).
I've had both and from similar manufacturers, both 30MM tubes, on the same power settings they are close enough FOV / Eyerelief-wise. Obviously you get 5 and 6 with a 1-6 and with that more mag, your FOV is going to be smaller than on 4, but that is just the way it works.
FOV: There is a major difference between 4× and 6×. I highly recommend you check the difference between them on the specific scopes being considered.
The reason FOV at 4 vs 6 becomes important is when you make a quick full throw adjustment (some scopes have a lever to aid this change) from 1× to 4× or 6×. The difference may impact how quickly you can regain the target or aquire others. Though most optics will list XX feet@ 6X Power setting: this is what you can view without scanning (Sample 19.2ft). At a 4X Power setting you have a wider peripheral view (Sample 36ft, almost double the 6×) without scanning but less magnification over all. If your not shooting against the clock or things that shoot back, it really does not matter as you have time to adjust magnification and find the target. The size of the kill zone obviously matters, say that of a man vs a squirrel. When I consider FOV, I divided it by 6ft man for perspective.
Side note: Just as we practice our handgun draw, one must also practice shouldering and transition of eye on target to eye and scope on target.
Jwlineman just switched from 4x to 6x for 2gun. He should have some good insight.
As for me, I've been running a 6x for a while, and while I usually keep it on 3x or 4x, the extra magnification is a game changer at 350 yards.
6x is usually more expensive, my Leupold is very light, so expense is really about it. However, if you actually shoot far enough, or small enough, there's no substitute for magnification.
JMO.