커피를 마실래요? 커피를 마실까요? I do not find any difference in the meaning between the two. These are exchangeable practically.

Both have almost the same meaning. But remember, you wouldn't use ㄹ/을까(요) to ask someone what they want to do if you are not doing with them. Ex : 커피를 마실래요? Would you want to have coffee ? (Asking if the listener want some coffee, doesnt necessarily mean the speaker want it to) Or Shall we have coffee ? (If you are in group and refering to group context) 커피를 마실까요? Shall we have coffee ? (The speaker is asking if the listener want to have some coffee together with the speaker)