Jim, the main difference, other than safer heads, between boomless and boomed sails is that the boom improves the sheeting angle. You can put the sheet somewhere convenient and still have a good angle. Without a boom you need to have the sheet in a more specific location. Depending on the type of sail rig and the type of boat, the geometry may actually be impossible. For example, a boat with a narrow stern and a spritsail that extends far enough back may not be able to provide the required sheeting angle for the sail. But when it all works out it's very convenient. Laszlo  

   Other considerations, one good and another not as good.

The lug rig with a boom is self vanging so the sail will keep its shape whether the sheet is let our or in tight. this is becasue in the area of the sail before the mast the luft is in front of the downhaul, giving tension to the luft which tension is then is connected through the boom to the entire length of the sail all the way to the leech. without a boom that will no longer be the case. this isnt fatal but it will affect sail shape off the wind somewhat. This combines with the fact that as you go off the wind more and more, your sheeting angle to the boat (without the boom) will be less and less optimal. so all in all, performance off the wind will suffer.

But on the good side, aside from the boom-on-head issue, having no boom opens up some additional advantages such as being able to run a line from the clew up to a turning block mid mast and then down and so when you want to drop the sail for a short break you can just snug it up to the mast like a furling sort of thing instead of dropping it.

I am going to give both, with and without boom, a try this summer (if I can get all my work done) and see which way I prefer it.

Boomless lug sails are nice because they're quick and painless to set and douse, and literally painless in tacking and jibing.  

Unfortunately the stock Northeaster Dory lug sail is the wrong shape for conversion to a boomless rig.  You'd have to secure the tack at the mast (technically converting the sail into a "standing lug"), and this in turn would rotate the clew high up into the air.  The sheeting point would move to a point 15 inches aft of the transom!

A loose-footed lug sail would be a shipshape option for casual sailing, especially if you do a lot of rowing (because it's so easy to stow the rig).  Changing the proportions brings the sheet lead inboard, though because of the narrow sheeting base in the Northeaster Dory, you're still going to have a lot of twist and inefficiency off the wind.  

 Loads more on lug rigs here.

 The link, "Loads more on lug rigs here" is great!  Wish it was easier to print.  The WB cover of Kerr's Caledonia yawl was Mar 2010 - gorgeous boat