What's difference between positions of these pedals? I'm playing more post-rock music, with lots of effects... still can't figure out where should I place reverb.
I'd say the more traditional approach is delay -> reverb. Reverb being used in this context to create "space" and delay being used to, well, repeat things (as opposed to using the delay with a short repeat time to create a bigger sound). An example of this approach would be something like Godspeed! You Black Emperor or even U2 -- where there's ambiance to the sound, but the repeats are distinct and defined and easy to pick out.
But running reverb -> delay is also a valid approach. In this configuration you're going to get huge washes of ambient texture that you can mix in, underneath your dry signal, to create an almost pad-like texture that follows what you're playing. For an example of how to use this technique effectively on guitar check out Sig Ros' Glosoli.
You really do need to experiment with the position of these pedals to find the sound that works best for your particular approach to music.
It theoretically does not matter. Effects that are "linear" do not depend on order. Both reverb and delay are linear(in fact, reverb is a type of delay).
Therefore the issue depends on the details. Pedals are not perfect and some different orders could potentially produce different results... but generally be close.
Here is a scenario where order matters. Suppose you have delay then reverb then tube amp. Suppose the delay has too much gain and overdrives the reverb pedal. It will create distortion and most likely not sound that great. If you swap them then the effect will be much better because there will be no unnatural distortion. Distortion is a non-linear effect and essentially it throws a kink in the equation.
i.e., linear effects are linear only up to a point. Guitar amps are generally linear but if you overdrive them they become non-linear(and consequently putting your effects before the amp or in the fx-loop then makes a huge difference if you are overdriving your preamp)).
This is why people say "experiment" and why I'll repeat it. There are too many unknowns to get a precise answer. Theoretically it doesn't matter, but we don't live in a theoretical world(else things would be much easier).
You should experiment and put it in a position that creates the best sound for you.
Neither is right or wrong, but I think it's worth discussing which is the most "natural".
Delay is an inherently unnatural effect. It's not often you encounter echoes in nature. Just a few places with flat rock faces at the right orientation and distance. In those places, the degradation with each repeat is significant. Tape delay has degradations, but they have a different quality. Digital delay has almost no degradation, apart from a reduction in volume with each repeat.
Reverb is something we encounter in real life all the time. Just clap your hands in a church or a sports hall to hear it.
So, chaining delay into reverb, is going to simulate the sound of playing through a delay pedal alone, in a lively reverberant room.
Chaining reverb into delay is going to produce something that couldn't be achieved in a reverberant room.
Which of those is "best" for a particular purpose, is entirely subjective.
I'd go for the approach that they are non-linear and it doesn't matter in principle, they would sound the same if they were 'perfect'. In a argument about natural acoustics you could equally argue that 'echoes' come after the reverb as the other way round.
However, this applies only to at least one being a perfectly clean effect.
e.g. a lofi type delay, or a delay with hi cut, will be then smoothed out by reverb that sounds hifi. but lofi delay last it might 'degrade' your reverb. as linear/nonlinear goes, EQ, distortion(tape/analogue delay), modulation are linear and more often than not applied to delay/verb.
if delay is last then it has to replicate the verb too which only makes one tail. the otherway round the delay only repeats guitar, but the verb makes a tail for each delay. whatever extra effects contained will be more prominent in the last one. and the processing power might affect things leading to clutter-analogue delay last=no hifi verbs...
in the case of modulated delay, or any modulation effect, the oscillation gets smoothed together and looses its intensity when before reverb, but might give a more subtle depth as there will be each phase of the modulation 'on top of eachother'. afterwards is would be much more pronounced and the oscillator would be an audible wave applied to even the verb tail.
buffers and tonality of the effect/dry path affect things subtly and ime the preffered one should go last if there's little other reason for another order.
I also think delay before dirt is overlooked. you can get clutter if the whole signal is distorted including delay- it can approach white noise. delay before can mean that later repeats are driving the OD less and remain clean, retaining some spacial effect and acting a bit like a parallel effect with od one channel, clean delay the other.
Then we get into more looper/delay stuff, in which case do you want to overdub a lot of different verb FX or do you want to leave repeats running and play with the reverb knobs.
At a push, delay could be seen as a reverb with just a single reflection - or reverb could be seen as a very fast delay with lots of taps and high feedback. So either way round, in a way you're stacking two similar pedals for a bigger effect...