You do get a signal strength indicator with the 1522. It can be viewed from the site survey mode. Depending on the router you are bridging it to, you will also get an indicator of strength from the router.
I'm running a 1522 as a bridge between my house and my neighbors. I don't have any issues with connectivity but do have a weaker signal than I may get with a 1555. I currently have a signal strength of 58% as indicated by the 1522 and it connected to my router at 54Mbps.
The 1522 doesn't have the external antennas, as you stated, but does have the 1000Mbps switch built in. If you plan on transfering large amounts of data between the systems on the switch, I'd go with the 1522.
The 1555, with the built in antennas, should have better signal strength if you are going a long distance or through a lot of walls. But it is limited by a 100Mb switch for the devices connected to it.
There was a discussion between the two here on the message board: http://forums.dlink.com/index.php?topic=2091.0