Well its all upon u...u r the one who is going to decide what u want to go for u can only learn about both of the bikes here but dont ask for choice of anyone else coz it will not necessarily be a good choice for u. for start u can learn about both by thoroughly reading the following threads:
For Piaggo
https://www.pakwheels.com/forums/bikes-3-wheelers/119293-ravi-piaggio-storm-125-euro-ii-bike
For GS 150
https://www.pakwheels.com/forums/bikes-3-wheelers/128515-suzuki-gs-150-best-bike-pak
https://www.pakwheels.com/forums/bikes-3-wheelers/13695-suzuki-gs150cc-gs125cc
https://www.pakwheels.com/forums/bikes-3-wheelers/138353-suzuki-gs150-photos-comments
For piaggo u can dig up the above thread but for gs150 i m quoting the most comprehensive review and advice for new buyers by "asad01" u should read and follow that criteria, i m sure that it will really help.
@asad01 Plz forgive me for quoting ur text without ur permission
Now if u ask my personal opinion than i would (and still) have gone for gs150 coz i am not into "looks" i need high performance and a robust machine which i found in gs with a weakness (NO Front Disk Brakes!!)thus i made my purchase and i m quite happy with it but than again its just me and i m not a legitimate source for recommendation coz so far i drove it for 200 kms only,
Finally for making ur mind consider:
1-Your usage
2-After sales service(Parts & labor availability)
3-Snatch Hot (consider this if u r in karachi)
4-Fuel consumption
Best of luck with ur purchase