Here is my opinion:
Insteon is probably the best you are going to get (at a cost of course). They are reliable, very fast and perform very well. They are also very well supported in LMCE. They are the most expensive, however, and I'm not crazy about their selection of devices.
ZWave is kind of a middle guy here. They are pretty fast, fairly reliable, and still pretty expensive. They have had a lot of integration issues in the past in LMCE. Their selection of devices is decent though. I havn'e messed with them much, but it appears that you need separate interfaces for their lighting vs. their climate devices (can anyone confirm this?)
X10 is also an option. Though they are slower (about 1 second latencies, sometimes a few seconds if a lot of brightening/dimming events are being sent), less reliable (though I swear that in about 4 years, I've never had a signal fail to go through. But then again, I do have my 2 phases coupled properly, which a lot of people don't do. They are dirt cheap (about $5-$7 each on ebay for switches and outlets, much cheaper if you buy a used lot). This is still my favorite as it has a nice "bang-for-your-buck" effect. I also like their selection of devices (Simple On/Off switches that work with Compact Flourescent bulbs, dimming switches, motion detectors, wall outlets, plug-in light and appliance modules, etc..). I have well over 100 x10 devices in my home - i cringe at thinking how much that could have cost with the more expensive solutions... (Though I readily admit that Insteon and Zwave are much nicer than x10 - But - I can have all of the features in my home as people that make much more money than I do)
So you do have some options.
I would say that if you have the money, go with the best. If you only want to add a few switches, etc, go with the best. If you want a lot of devices and you don't have a large income to support it, i would definitely recommend the x10 - they aren't nearly as bad as some make them out to be)
Either way, you'll have fun. Home automation is addicting!
Actually, a Lamp module straight from ZWave is like 40 dollars US, and a lamp module for Insteon is 34.99 dollars US. I would say that they are both roughly the same cost. They both have an interface unit that needs to be purchased in order to operate the units. With that being said I would lean towards Insteon. It utilizes a meshed Wired and Wireless network and is inter-operatable with X10, if you wanted to use that as well.