I already responded but I will elaborate.
I learned Maple first. It does everything I need, I like it, and I have little reason to switch.
I tried Mathematica a bit recently, and I didn't care for it. This is why (these are opinions, of course):
The horrendous syntax (capital letters, square brackets, etc.). Maple's syntax is more intuitive, and easy to learn and remember.
In Mathematica you have to do Shift-Enter to execute a command. I had never used a programming language/environment where you had to do this, and I had no reason to think I would have to use the Shift button. This is embarassing, but it took me hours to figure out how to get Mathematica to do anything, like find "2+3". If you just hit Enter and not Shift-Enter, Mathematica does not give an error message, warning, or hint. I even did some Web searching, and none of the pages I saw mentioned it. Maybe I should have done a Mathematica tutorial first. Certainly students using Mathematica might have the same problem. Another respondent to this question stated that Maple has changed or will change so that you have to hit Shift-Enter to exectute a command. Please don't do it! You got it right the first time! In addition to the above reasons, currrently I need to use Shift-Enter much less often than Enter, so I would have to use more keystrokes (if you really switched or will switch, and I cannot "fix" it).
I prefer Maple's worksheet environment (I have never used the Document environment, which seems to be the default in new version(s) of Maple) to whatever Mathematica is using. On all the computers that I use Maple on currently, I get command prompts by default (I cannot think why anyone what not want command prompts) and in Mathematica I don't think there were any. In Maple it is easy to execute commands at will, cut-and-paste input and output, execute an entire worksheet, hide your output, and stuff like that. I never had to spend a lot of time learning to do all these things. Maybe if I did a Mathematica tutorial it would teach me how to do these things, but I never had to do one for Maple.
The only things I liked about Mathematica that seem to be missing from Maple are: (i) you can toggle between viewing what you have typed and a "pretty" math-formatted version of it; and (ii) the outputs are named Out, Out, etc., so you can use them easily. These features are not enough to want me to switch.
This is not really an answer to your question, but for my purposes, just about all the changes made to Maple since Maple 9 or so have just made Maple a little harder to use. A while back, Maple changed the default behavior so that what you type is instantly replaced by "pretty-looking" math formula. Unfortunately, this makes it difficult to see what you have typed. In Maple's own documentation and in worksheets people post on mapleprimes, I often find it difficult to figure out what I have to type to do a certain task. We no longer teach Maple (or Mathematica) where I work, but I encountered this problem once while teaching - I couldn't help a student fix her mistake because I couldn't tell what she had typed. It is not obvious how to learn how to "fix" this behavior without asking someone. One definite improvement is the blinking/matching parentheses. This is a huge convenience, and I think it has only been around for a few years.
I would venture to guess that a large majority of your users use Maple to do math, and are not interested in fancy things like writing technical reports or Web pages or GUIs, etc. If you invent ways for people to do fancy stuff, that's great. But I think you should make Maple easy to use for the majority of your users. I participated in a Matlab Webinar (more of a sales pitch) a few months ago and I saw a new version of Matlab. The desktop was cluttered with a huge number of buttons that most people probably don't need. Even if I bothered to learn what all those buttons do, they might change in a few years. I hope Maple doesn't go too far down that road.