Long or short handle bats really depends on your style of batting and situation of the match.
To put it simple, short handle is for stroke makers/ground shots and long handle is for big shots/heaves. This doesnt mean that you cannot have the bat for vice-versa.
The reason is obvious since with a long handle, you have a longer bat to swing and more probability of connecting.
Every batsman has bats of different sizes and weights which they wield on the situation of the match. If you have noted the batsman suddenly changes his bat in the middle of the innings, this might just be to change the tempo of his innings or may be he is not feeling the ball right.
Let me explain you my case, i played a pretty decent level of cricket back in India, (now in US also).
I have 3 bats, which i swap,
one regular handle but a bit heavier. I use this bat at the start of the season when my feet is not really moving and i just dont have control with my lift and swing. The heavier bat really help me check my shots. Again i prefer this bat when i play in turf wickets when the ball stops a bit and come to you.
one regular but lighter, which i use when in form, when i know i am hitting the ball well. when i really am in control of the bat and the swing. This i use in matting wickets when the ball comes to your bat skidding.
one long handle for swinging and swashing
Now you decide on your style of play and choose your bat. If you are batting down like 5-6 and want to be hitting from the word go, you might want to pick a longer handle bat.
cheers ! and best of luck