Ok, then I would definately go with .177. It is the only serious choice for any serious target shooting!
The HW100 is in a different league to the outdated S4** range which are clinging desperately to their early 90's titan heritage. Poor design, shot count and materials are its biggest problems, but they sell lots so many will argue. For exactly the same reason the Kia Ceed is better than an Aston Martin!
The HW100 is a great gun, but for around the same (used) price I would look at the Daystate Regal as well. Both will be more satifying to own than the air arms.