Oh this is a tough one because, as previously mentioned, both lights are *so* very different and are thus designed for very different purposes. Their only real similarities are that they both are from Fenix and they both are powered by two AA cells. I have been EDCing an LD20 (XR-E Q5 model) for six months now and I have had a TK20 for about a month. Both are excellent lights and I absolutely love them both. I think that there are a few simple questions that you might want to consider first before making a choice.
1. Are you going to EDC these lights? If so, the TK20 is just too big and heavy to EDC in my opinion, unless you carry it in a backpack or waist pack. If you're going to carry your new light in a holster, like what I do, then the LD20 is really the only choice here since it's much smaller and lighter than the TK20.
2. Are you using your new light specifically for outdoor use? If so, then the TK20 is the better choice as its neutral tint LED is vastly superior outdoors for things like night hikes. The LD20, as with all cool white LED lights, just washes out all colors while with the TK20 those colors just come alive. I've done direct comparisons between my two lights outdoors and every time I turn on my TK20, I put my LD20 back in its holster where it usually stays. The TK20 is *that* good outdoors in the wilderness.
3. Do you need flash modes? The LD20 has both an SOS and a strobe mode, while the TK20 has no flash modes at all.
4. How much do you want to spend? The LD20 is about $60 and the TK20 is about $50. Is that $10 a deal breaker for the LD20? Honestly, I think that the TK20 is one of the best deals out there for a high quality LED light.
5. Will you be using your new light outside in the winter time? If so, the aluminum bodied LD20 can be pretty cold in your hands if you're not wearing gloves. The TK20's sticky rubber grip feels *really* good when it's cold outside and it definitely does make carrying it much easier.
6. What is more important to you in a beam, throw distance or flood? The TK20 has a very tight hot spot that throws extremely far for a 2 AA light. The LD20 has a much wider hot spot that does not throw anywhere near as far.
7. What kind of tailcap switch do you like? The TK20 has a forward clicky switch, while the LD20 has a reverse clicky switch.
Those are just a few "tip of the iceberg" questions that highlight the differences between the two lights. Both are *excellent* lights though and I'm sure that you'll be happy with either one. Whichever one you ultimately choose, please feed it right and don't use alkaline batteries. The last thing you want is a battery leak in your new light.
Please let us know which one you decide to buy.