The short answer is that you are making a really great choice by going with templar. Since your well over 20th level hump half classes are going to give you much more overall "power" than the strait classes. Because we'll, you won't be stuck with one "trick" so to speak.
One of the big choices you will have before you is whether or not you want to go weapon heavy, or spell heavy, or cut it right down the line. If you go pretty even you could have 4 profs (swing 6's base damage) and a 4 column all the way through your 9th level spells. Alternatively you could drop off some of the spells and go for swinging 8's. Or only swing 5's with not so many weapon skills and perhaps have a 5 column or a 4 with 5-10 lvls or ritual. And in case you hadn't noticed ritual these days is sweet for combat with the new high magic.
As far as Celestial vs. Earth really I'd say that would strongly depend on how you like to use your magic. Because essentially Earth is supporting magic. You'll be able to memorize some offensive stuff like binding and curses, but you'll probably want a solid number of protectives and healing to keep yourself on your feet as well. And of course that is the beauty of a Templar or half class mele/casting class is that they can really be a full self service station. With Celestial you'll give yourself a much more offensive option with your magic. Even if it isn't all Evocation with a full 4 column you'll be able to throw 5's with your wand charges which can definitely come in handy. Not to mention you give yourself access to the best offensive/defensive spell in the game IMO, Prison.
Another thing to keep in mind that how you build your spell column might be something you would want to chose depending on the type of magic you pick. For Celestial its all about getting your columns and 9th level spells now. Since those augment your Wand damage and give you the best spells the Celestial school of magic has to offer. With earth it can actually be perfectly useful and handy especially for a half class to go wide base on their pyramid because the lower level earth spells can be really awesome for being a "combat medic" and just picking people up off the ground.
As far as the sweet spot for effectiveness that will depend a lot on how you like to play your game. If your more of a fan of the "beat them down" version of things then I might go weapon heavy, earth templar. If you like to mix it up a bit more and perhaps spell things down 2 or 3 times a day when you decide to use what you have consider the celestial templar option. One last thing to keep in mind longterm is that if you should ever get access to a Mercury Golem, the celestial templar option with 5 or 10 levels of ritual to store themselves with high magic into that unholy killing machine will make you one of the most powerful possible characters/classes on the field... Really only possibly second to the celestial Adept... But that's another conversation
Any more specific questions? That's my broad picture overview of some of the various strengths and whatnot of the awesomeness of the half classes. IMO once you are beyond 20th level with pretty much anything its really well worth it to go half class, cause lets be honest, swinging 20's really isn't "necessary" even if it is supper cool.
Oh and for the perspective of how people are leaning in Alliance these days Templar is getting more and more popular, and in many event breakdowns that I've seen recently they have outnumbered the Fighters.