What's better, a Meade Super Plossl, a Celestron X-cel, or a UO Ortho?

I think the nod would go to UO among those who know.  The Celestron and Meade eyepieces can be very good, but with the exception of the 14UWA you very seldom hear talk about the Celestron and Meade offerings in the amateur community.  Well OK, the ultima eyepieces from Celestron have a good rep especially the barlow.  The Tak LEs are not on your list but I think the Takahashi LE series is something you might want to look at.  Higher priced than your UO ortho but not sky high.   Used, the Tak LEs come up regularly on Astromart for around a hundred bucks.

You might call Teton telescopes and see if any of the Super Monos are left.  That is considered a "planet eyepiece."  The UO is too.  If it were my money and I had those three choices, it would go to UO, but with my money what in fact I purchased was a Pentax XW 5mm and a Super Monocentric.  These options are, however, in the high and low 2s, respectively.

Also there is a yahoo eyepiece discussion group.  I'll put it in the links.

I also always put in a word for astromart.   You pay a ONE time fee of $12 and have access to a huge used market.  You can pick up a used UO for about $25 less than the new priced but they tend to sell fast (which says something).  There are a number of discussion groups there including one which is JUST eyepieces.  That groups is VERY ACTIVE, the one on Yahoo is so-so.  There are other forums on Cloudynights.com, which is free, but I can't emphasize enough how important astromart is and well worth the once-in-a-lifetime fee of $12.  Cloudy Nights has used market too but it is 1/100th the size of Astromart and some people selling there got booted out of Astromart for various forms of misbehavior.

I'll put in a link to Teton telescopes too.  I am not a big fan of their "overall philosophy" which is heavily biased (big surprise!) to the products they sell (I prefer other vendors like Highpoint Scientific) but they DO have the super monocentric eyepieces which come out only in very limited editions.

Most of the people who CAN'T get a super mono would go with a UO ortho.  

In this price range I'm an enthusiastic supporter of the Edmund Optics RKE but their shortest focal length is 8mm.

You really want to get into a discussion forum where people can do the give-and-take without getting into this best answer nonsense.   There are lots of different views and it helps to hear a bunch.

5mm is a short focal length and "best used" with a telescope that has a drive.    Most dob owners would get some kind of wide field.  The Baader Hyperion eyepieces are an option as are the TMB-Burgess "planetary" eyepieces--but I would get a UO over any of those choices.  The Hyperions are available at OPT (oceanside photo) not astronomics; be careful, the coatings on the look-alike Stratus eyepieces by most accounts are not as good.

Oh yeah the excelsis web site has a LOT of "user and voter reviews" of equipment including eyepieces.   A very useful resource even if it does have a flawed voting system.

You should be able to look up all the brands I've mentioned on the astronomics web site.  It's a huge operation, but when possible, I buy from Highpoint Scientific, a nice small outfit where there is a lot of knowledge and they're not "pushing a line" like Teton.  

As you spend money on eyepieces you will discover that folks will chat on and on about the nuances of all of them but the fact is that we're stuck under a big bucket of slosh called the atmosphere and there's no real wrong way to go.

Hope that helps,
GN

The UO ortho is by far the best of the three; the others are just run-of-the-mill Chinese Plössls rebadged to Celestron and Meade. All of these will have extremely short eye relief, so you might do better with a 10mm UO ortho combined with a quality 2x Barlow. If you can spend more money, get a modern high eye relief design like the Tele Vue Radian. I have a Radian 5mm, and its my most used short focal length eyepiece.

Before you decide on buying a 5mm lines figure out which lines you can use with a Barlow lens to achieve the same power.
If your telescope can hold two inch lines, I would suggest you get a 5mm Celestron, or and Orion.
One more thing, you probably will only get good viewing in real cold weather with that much power, that is because there isn't as much moisture and dust in the air in cold weather.