Planars/ Electrostats benefits over box speakers?

A big benefit of panels for me is one that isn't talked about enough: Full size images! A grand piano is HUGE, and a good recording of one sounds that way through panels. Through most boxes they sound miniaturized. Panels also create a soundstage you are looking up at, rather than down on. The best deal in panels is the Eminent Technology LFT-8b. They aren't that big, 1' wide by 5' tall, and match better with tube amps than do Maggies, being an 8 ohm load rather than 4.

It's hard for me to generalize on which type of speaker (planar vs dynamic) produces better timbre, because there's a ton of variation on that front within both types. Some people believe that planars are "faster" and offer more detail, but I wouldn't generalize and would again argue that there's a ton of variation IME in the perceived "detail" delivered by stats vs planar magnetics vs ribbons, and (to a lesser extent) among different designs within each type of planar.

If you're looking for an advantage that's generic to planars, I'd agree that it's mostly related to the dispersion characteristics. The reverberant field usually sounds richer and more natural to me, possibly because there's less variation in horizontal dispersion vs frequency in most planars vs most dynamic speakers. Imaging also differs qualitatively, as does sound staging. However, personal preference is likely to determine which presentation you prefer on those fronts.

I also feel that dynamic designs have some common advantages (bass impact, for one), so it's back to trade-offs.

Have fun with the search.

Having owned both Maggies (1.5's and MMG's) and Martin- Logans (Aerius) I feel compelled to chime in. Without getting too technical about how they produce sound let me say that their biggest advantage over box speakers is simply NO BOX. They both produce such a natural uncolored sound that when heard for the first time it can be very unsettling to many an audiophile (me included). So much so that some people will simply say "I gotta have these". However, if hard driving power Rock is your thing then please look elsewhere. But for softer Jazz or Steely Dan and such I can't think of anything I'd rather listen to them through than a set of Maggies. I currently run Ohm's simply because they handle anything and everything nicely but if I ever have a third music room I will definitley have some 1.7's for those late night Jazz listening sessions, anyway just my 2 cents...

I also like Marty's post. Another advantage is there are no cabinet colorations caused by resonance and reflections.

Small panels have less bass and often employ a dynamic driver that is difficult to integrate in to the sound of the panel. Although I think Sanders does a good job at it, the best I have heard are the large electrostatic speakers. Those have phase advantages; i'm not sure if cone drivers can be as good in that area. To get first hand knowledge of panel speakers listen to more brands than Magenpan and ML.