A320 ARS vs Manual Flap Retraction
 

It is recommended to use Airbus Auto Retraction System (ARS) on A320/321 for a heavy weight takeoff with speed constraints like 220 knots in SID. My question is suppose I am going to takeoff in 1+F config. Now just by looking at characteristic speeds like F, S & O and climb constrain speeds is it possible to decide if one manually retracts flaps at S speed or let ARS retract flaps and then manually retract slat after climb constrain point is crossed. In short, is there a way to decide ARS vs Manual Retraction based on numbers?

Yes, if your green dot speed is bellow the speed restriction, you may retract to 0 at S speed.

Indeed. If the entered ZFW and FOB are correct, the MCDU will calculate green dot speed quite accurately, which will give you good indication wether or not the maximum speed constraint can be flown clean or not. For example, if green dot speed is 225 on the MCDU but there is a 220 max speed restriction, then you know you will not be able to fly the departure in clean config till you pass that speed restriction.

Now wether you use ARS or not is a separate question. Personally I regard ARS more as a finicky gimmick in this scenario. The same can be achieved by selecting 200 knots in 1+F and retract the flaps on schedule after passing the restriction. You don’t need to fly the MAX speed, so why bother?

The thing with ARS is this. Yes, ARS will automatically raise the flaps passing 210 leaving only slats out. In this condition you can fly at 220 knots and it generally works out fine. However there is an increased risk of overspeeding later on when you retract the slats since there is only a 10 knot margin to the overspeed. Add in some turbulence or an early level off during acceleration and you will pass 230 knots before the slats are fully retracted leading to a slat overspeed, as a few of my colleagues have experienced :}