B737-400 HOLDING AT TERRI INTXN 6000 FT. CLRED TO DSND TO 5000 FT AND VECTORED FROM THE HOLDING FIX. ENCOUNTERED MODERATE TURB AND ELECTED TO HAND FLY. OVERSHOT ALT 300 FT. NO COMMENT FROM CTLR.

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HOLDING AT TERRI INTXN; DUPONT 3 ARR. WX; IMC; MODERATE RAIN; ICE PELLETS AND MODERATE; OCCASIONALLY SEVERE TURB. WE WERE NEXT TO EXIT HOLD FOR APCH TO RWY 9R; PHL. ATC ISSUED DSCNT FROM 6000-5000 FT. WHEN JUST LEVELING AT 5000 FT WE ENCOUNTERED SEVERE TURB. I ELECTED TO TRIP OFF AUTOPLT AND HAND-FLY ACFT FOR MORE POSITIVE CTL. DURING THE TRANSITION FROM AUTO TO MANUAL FLT; I INADVERTENTLY DSNDED TO 4700 FT MSL; A 300 FT DEV. THIS HAPPENED DURING THE MOST SEVERE PORTION OF SHEAR AND TURB. I QUICKLY CORRECTED THE DEV AND RE-ESTABLISHED AUTOFLT. THERE WAS NO ATC CONFLICT. I FELT THAT DISCONNECTING THE AUTOPLT WOULD ALLOW BETTER CONTROLLABILITY OF THE ACFT DURING EXTREME TURB. IT SO HAPPENED THE TRANSITION FROM DSCNT TO LEVEL FLT; OCCURRED AT THE SAME TIME WE ENTERED THIS APPARENT SHEAR ZONE. I ALSO WENT TO MANUAL FLT AT THIS TIME. I NOTICED THE DEV FIRST; AS THE CAPT WAS TALKING TO ATC. THE DECISION TO FLY THE ACFT MANUALLY; OR REMAIN ON AUTOPLT IN SEVERE TURB IS THE QUESTION IN THIS EVENT. ALSO; SHOULD THE PLT LET THE AUTOPLT CTL THE ACFT TILL IT DISCONNECTS ITSELF? FROM PAST EXPERIENCE; I BELIEVE I MADE THE BETTER DECISION TO HAND FLY THE AIRPLANE.