Just wondering if these two features are really necessary for a full review experience... there is no difference between the two, is there? Has anyone tried selecting just one?

They're the same in terms of what is revealed when you reveal the answer, but I keep the, seperate because sometimes I remember the pronunciation of a character, and not the definition, and vice versa.

I did turn off the tone test though, because I never mark the reading right if I did not remember the correct tone.

I don't use 'reading'. Getting enough review as it is already.

I don't use reading. I often consider dropping tones and using the definition prompt as a catchall. It seems especially weird for me to separate the tones from the reading.

I can see value in the 'tone' test for those who previously learned vocabulary in such a way that their tones are not very solid.

Don't use either. Writing (with hidden pinyin) and tones cover all I need.

for the first 1000 characters i havent used reading but now i started. and its a totally different feeling!

Good discussion. I decided to just keep on writing (hidden pinyin), and reading (to test pronunciation and definition).

I also decided to drop a prompt. I only do writing and definition now. It's great.

I keep them all. The more I'm tested the better in my opinion and I can concentrate more on ways to remember each character.

I had never thought to turn off any of the prompts. I'm down to two now, and I must say that I find it glorious. But I guess I'll have to keep myself on a short leash not to ignorer tonal mistakes.

Will testing tones and reading separately, or even testing reading/tones and definitions separately, be more beneficial than testing them together? One of the well-known SuperMemo rules is to make cards as simple and basic as possible, which would suggest separate testing. On the other hand, in the real world you need to recall both reading and definition together so conditioning yourself to only focus on one or the other might be a hindrance.

Anybody know of any relevant research?

You'll probably have to try this one for yourself--there aren't really any well-studied systems that combine the reading, tone, and definition reviews (or sometimes writing and tone reviews) while also tracking the knowledge separately and sometimes thus prompting them separately the way the Skritter web app does. (The reason is that it's crazy complicated, which is why we skipped prompt combining for the iOS app.)

My guess is that testing them together on Skritter is more efficient SRS-wise, because the parts that you miss will then be pulled apart for further focus during easier reviews, but you get a more efficient, combined harder review without having to worry about intermediate spacing effects between different prompt types. But there's also the cost of context-switching when you switch between prompt types, so with a lot of reviews, you might want to do one prompt at a time just to go through them faster.

So I turned off tones and definition and it has been gratifying for the past two weeks. The more characters I learn, the more context I seek and the more interaction I need with each character. While having the 4 prompts my reviews tended to be quite mechanical, even addictive at times. I spend more time with each prompt now (writing and reading) but I definitely feel that each character is becoming part of a greater picture as if I'm more actively involved in my progress. I'll try this week to do the 4 prompts though, just to see what happens. It's been interesting reading your comments, thanks for the input.
春节快乐!