1. Due to time constraint we have to go on a Saturday. Will it be very crowded?
2. I don't mind paying for a gold flash pass but will it save us waiting time? Read in one of the reviews that flash pass doesnt make much of a difference.
Even if it's sunny and nice, crowds aren't too bad on non-holiday weekends this time of year (remember, MM is closed on weekdays until late March, except for the two weeks around Christmas). I wouldn't bother with a FlashPass at MM until the summer.
As always, arrive in time to enter the park at the official opening time. There are various plans of attack, but the simplest ones are either go left around the park starting with X2 or go right, starting with Goliath.
What I would recommend you do is get there at opening and ride X2 and/or Green Lantern (both are upcharges with Flash Pass), then check out the crowds at the other rides. If most of the lines are in the 10-15 minute range, don't bother with Flash Pass as it is unnecessary. If, however, the major rides have waits closer to an hour, you might want to consider it. Get the Gold Flash Pass (the regular doesn't do much) and enjoy the park at your leisure. This past summer, I used the Gold pass at SFNE and SFGAdv, and at both I was able to do everything and get a number of rides on the best coasters, all of which had 60-90 minute waits for the entire day, all without rushing around to hit everything.
It's up to you, and you can judge based on crowd levels, but good rides to check for crowds are Tatsu, Goliath, Riddler's Revenge and Apocalypse. All of these have obvious merge points, and if the line is not backed up behind these Flash Pass will do NOTHING! Either way, you will have a great day, and if you think you can do everything without one, you might as well save your money.