I just went through this process too (deciding between FPX Elite and Montecito). Currently waiting to get an FPX Elite 36 installed. Decided to go with FPX instead of the Montecito because of the features it had, including:
****** I have the FPX 44 as well, 4th winter heating with it. Thought I would add comments to Woodrows excellent post. Note - I know nothing about the Montecito unit so this just applies to the FPX
1. The FPX has an external blower that can be mounted outside or under the crawlspace etc a good distance away from the fireplace. This should keep the fan noise substantially reduced to almost nothing. The Montecito has a fan built into the unit so it's going to be much louder. Personally I hate it when sitting by the fire and you have to hear the fan whirring all the time. Especially as it gets older and noiser. Harder to repair/replace too. I much prefer a remote fan with almost no noise.
********* Fan noise on the FPX is not an issue. I have the fireplace in our family room where we watch TV and dont have to adjust the volume w/fan running
2. The FPX has a cat, which allows slower burns and more heat while in slow burn mode. Also should reduce overall creosote production if slow burns are performed regularly, making cleaning easier and less frequent. Cat is expensive and has to be replaced every 5-7 years, but decreased wood consumption and less frequent cleanings should way more than offset price of the cat. Secondary air burners (as used in the Montecito) can and do go bad too, which also costs money to replace (could be warrantable, depending).
******* I recently sent an email to Travis asking for indicators as to when the CAT should be replaced. the reply "The combustor will only need replacing if it is crumbling to the point that the exhaust cannot go thru it. As long as the holes are unobstructed the combustor will work great." I will add that after 4 years the Cat seems fine.
3. The FPX Elites use ordinary firebrick in the firebox. If something happens to them they can easily and cheaply be replaced from a wide variety of suppliers. The Montecito has cast, textured refractory panels that will be much more expensive to replace if they get cracked and will probably have to be special ordered from a Lennox dealer only. As the stove ages, you run the risk of them discontinuing parts like this for it.
4. The FPX uses a much less expensive air cooled chimney. Some people have warned an air cooled chimney allows cold air to penetrate into the fireplace and seep into the house when the unit is not in use. But the FPX instruction set has a "P" trap setup that supposedly eliminates this problem (see instruction manual or product brochure). Hoping that's truly the case.
**** no issue with cold air seeping in
5. The FPX uses a positive pressure system (i.e. it takes outside air and blows it into the heat exchanger before going into the house). This system keeps the pressure of the heated air inside the house slightly higher than that outside, which helps keep the creepy coldies from seeping into the house via cracks and small leaks. It also tends to pentrate the heat throughout the house better than a standard, non-pressurized convective system. From what I've read here and from a technician that installs both the Montecito and the FPX elites, they say this "POS" pressure system works well (the technician said they're both good units but he would opt for the FPX if he had to choose one for himself). The POS system also introduces fresh air into the house, which some people like because it can help keep the inside air from getting stale (and even oxygen deprived), which can be a potential problem with newer, more airtight homes without a heat exchanger system.
**** POS works! I am in process on upgrading windows. I still have a few that are drafty - when the FPX fan is running there is zero draft anywhere in the house. Does a very good job of circulating the heat - I took a candle and held it in all the door ways - clear to see.
6. Both units look great and have terrific capacity, so it's a wash there.
7. The only feature I truly like about the Montecito better than the FPX, is apparently it functions pretty well as a heating device in the event the electricity goes out. If you use the big heat vents it has, it will passively output a large amount of convective heat without a blower working. But personally I think the Montecito's heat grilles are ugly over the fireplace, plus I have a generator. Also, I'm not sure the FPX wont put out decent heat in a power outage either. Just heard the Montecito has an edge there.
**** Luckily we dont get too many power outages but did have an extended outage in the freak october storm. Ran the FPX and it did provide some radiant heat but clearly would not have been enough during a real cold winter power outage.
8. The Montecito does have a better warranty than the FPX. Although after reading it, it's pretty legalistic and sounds like there's wiggle room for much of what's "covered."
I notice you live in Idaho, I know it can get really cold there. If you really want to heat your house, you might want to consider a stand alone stove instead of these fireplace units. Generally I think fireplace units look better, but a high efficiency stove is even better than these fireplace units for heating efficiency. If I didn't live in a temperate zone and lived near the rockies intead, I'd go with a nice modern soapstone stove like an Equinox or the Woodstock Progress especially. If you can stand its appearance, the Blaze King KING model seems to be generally regarded as the very best.
Good luck!