I have used Lithium packs for more than several years, puffed them, blew them up, tested them to see what their limits are in the context of RC planes. At one time, I was carrying more than 9000MAh on my GP 1/4 scale Giles just to amuse myself and get great reaction faces from my flying buddies. The cells were 3 sets of 7.4v at around 3000MAh each and regulated to 5.4v.
A few of things I noticed. First and foremost, Lithium technology failure results in an OPEN circuit. Zero volts. This is akin to NiMh technology but NiCd shorts closed so if a cell in a 6v pack dies, you still get 4.8v. That's why I double up on my rx and servo batteries.

From what I have experimented with, while both LiIon and LiPo batteries are both great, the new LiPos can deliver much more current than LiIon cells. LiPo cells can now deliver 30C or more if needed without killing itself. The most I was able to get from LiIon cells were less than 2C...more than that and you're looking at an early demise. Saying that, I make packs that are 3000MAh 7.4v so I can safely get 3 amps.

Also, LiIon cells can't seem to take "abuse". Drop one from about shoulder height and you'll see when you try to measure for voltage. Zero. When broken as in a crash, the do sometimes catch fire. This is unfortunately, first hand knowledge :-( They are basically high pressure vessels that is held tightly by the casing.

Even though I may have "bad mouthed" LiIon batteries, I still use them because I have several packs already made up. I just double up on the cells and encase them in foam inside the plane. Also, if you're a DIY guy, you can "liberate" excellent 1100MAh batteries from Canon packs. Right now, my Canon 300D has an external battery pack that is 7.2V, 4500MAh made from the infamous Sony cells from Notebooks. You can also get good cells from "bad" notebook packs. Usually, notebook packs are 3S3P, 10.4v at xxxxMAh. When they go bad, often you will find 6 or more good cells if they are not dropped. I used to do this on old, "broken" Toshiba packs. There's a plus side too. These packs usually come with a built in charger although you need to have some soldering skills.

When I make my packs, I run a continuous test of 1C charge and 1C discharge for about 10 cycles just to make sure I didn't thermally ruin the packs and whatnot.

Given a choice, and money, I think I would prefer LiPo packs only because they are lighter and can handle physical, electrical, and thermal stresses better. I will still use 2 in parallel for safety.

Unfortunately, the question in this thread is NiMH vs NiCd. I prefer NiMh only because it is a proven technology, much lighter than Nicad (by capcity) and does not develop memory. Then again, Nicads generally handle more current better. I said generally. I haven't used NiMh and Nicad in a few years except in F5B planes. More power can be applied to digital servos as needed without dropping the voltage to unacceptable levels. I said generally. I haven't used NiMh and Nicad in a few years except in F5B planes.