I use weighted decision matrices (I actually just used one today to figure out what project to pursue).
Start off with a table, and to the left side draw one column and title it factors. Then make additional columns to the side for every idea you have. Now write down all the factors that are important to you, e.g. How big is the market, how fast can I finish the product, how easy is it to market, etc.. After you've listed all important factors, weight each factor from 0-10, 0 being it is not at all important for me to 10 being it is very important. For example, if market size was pretty important to you, you would have one of the rows in the factors column with the value of "Market Size (7)". Add weight to all factors.
Now for each of your ideas, go down the column and grade each row, 0-10, on how well it performs on the given factor. For example, IdeaX has a very large market so I will put a 10. IdeaY has a large market too but somewhat smaller than IdeaX so I will grade it 8. Do this for all the ideas on all the factors.
After this, you must grade each factor overall (weight * grade). Since we said market size weighed 7, and IdeaX had a grade of 10, IdeaX scores 70, while IdeaY gets 56. Do this for all the ideas.
Now add up all the scores of each idea and write them at the bottom of the columns. Whichever idea has the highest score is the winner.
Factors IdeaX IdeaY
**********************************************
*Market size (7) * 10 [70] * 8 [56] *
*Finish Time (9) * 5 [45] * 9 [81] *
*Mrkt Diffic (4) * 8 [32] * 3 [12] *
RESULTS: 147 149 -- IdeaY is the better choice
// Note that marketing difficulty and finish time judge how easy / quick it is to do said tasks.
// Higher value means easier / quicker.