In general more rounds are better, up to the maximum of 10+1
BUT
For a beginner, what matters is the gun that you can handle, operate, and shoot with the greatest safety; the one you are most used to.
If you take the 3906 with both 8 and 9 round magazines, you will be limited to 8+1 because all magazines must be loaded the same.
You need three magazines, carriers for two, holster for the gun, eye and ear protection, and a concealment garment.
I suggest you spend the next week practicing drawing from under that concealment garment DRYFIRE so you don't get tangled up when the gun is loaded at the match. (Some places don't require beginners to wear concealment, some places don't require concealment in hot weather.)