My first (not finished yet) app, http://webworkschedules.com is a large web based project that has been a lot of work. In the middle of it, I split off for what was suppose to be a two week side effort to create my customer support knowledge base app. It has been a more than two week detour. In that time period, my effort on web work schedules has been minimal.

So my recommendation is that you focus on the quick easy one first and try to sell it, for the learning experience. There is a lot more to do after the code is written and having done that once will make it a lot easier the second time around. Once you have that done, then pick between the two large projects. From my personal experience, having the consulting work on the side has been a huge plus for mental relief for myself and my family. While it is taking up a good chunk of time, it gives me financial security to work on my uISV.

I would not try to split your time between projects. My guess is that as an engineer, you will favor the project with the fun items to do. Then switch to the other project once its tasks are more interesting. In the and you will end up with several partially finished products with nothing to sell.

Also if you finish your small app first, you can use it as your fall back to work on if you get stuck on your second project.

Good luck!