A few nights ago I compared the Vortex DB 15x56 with the Pentax 20x60, both mounted. Tonight conditions were again really super: the moon had set, virtually no clouds, and I stepped out several hours after a short, intense downpour that cleared the air. I spent about an hour on the back deck in a zero-g chair using my OB 15x70 Ultras and the 15x56s handheld. The results were mostly what you would expect, but with a couple of significant observations on usage.
Ptolemy's Cluster (M7) was visible to the south, just above the treeline. This object is like an old friend, appearing this time of year but so low in the sky the conditions have to be very good to see it well. In the 15x70 M7 was absolutely sparkling; it was good in the 15x56 but not as pleasing. M8 (Lagoon) and M20 (Trifid) were both pleasant in the 15x70, M8 was good in the 15x56 but M20 was a bit dim. The Small Sagittarius Star Cloud was really spectacular in the 15x70, good in the 15x56. M27 (Dumbbell) was easy to spot in the 15x70 but very faint in the 15x56. In Cygnus, you could see individual stars in the Cooling Tower with the 15x70 using averted vision but all you could get in the 15x56 was a "perception" of individual stars. Finally, panning through the star fields in Cygnus was very nice in the 15x70s; it was about the same in the 15x56s, just a little dimmer.
Both the 15x70 and 15x56 have similar fields of view, but there is more eye relief (18 mm) in the 15x70 porros and it makes for a more comfortable view. The 15x56 roofs have a few mm less eye relief and it shows, both in a truncated TFOV and in slightly more fussiness about eye position (I wear fairly thick eyeglasses). Then there's the difference in weight. The 15x70s weigh 5.5 pounds, which makes them harder to hoist for long periods. However, their weight also damps out a good portion of the shakes when using them handheld, especially when viewing at high elevation angles. At just over 2 pounds, you can hold the 15x56s more easily for longer periods than the 15x70s but you'll get more shakes, noticeably from your heartbeat.
As I said earlier, these results are mostly what you'd expect, i.e., the 15x56s put out decent but dimmer images than the 15x70s. I wouldn't want to give up the 15x70s, but the significantly lighter weight of the 15x56s means I'll likely reach for them more often for shorter, handheld sessions.