I have an STP and it is great for what it is designed for: dirt jumps, urban assault, etc. However, it is terrible for xc. Granted, you mention you don't sit down, so the lack of seatpost extension may not bother you. But the bars are way low for xc riding, and the top tube is too short. So, in my opinion, if you want a trail bike, stick with the F8, and get the STP if you want something for dj, urban, fr, etc.
As for strength vs the F8, the STP probably is stronger since it is designed for jumping whereas the F8 is a pure xc bike.
You could probably put the longer fork on the STP without voiding the warranty (you'll want to double-check with your local Giant dealer), but that much travel will really slack it out, since it is designed around 80-100mm travel. If you want something a little longer, I'd go with 120mm, maybe 130mm at longest in order to keep decent handling.
As for strength vs the F8, the STP probably is stronger since it is designed for jumping whereas the F8 is a pure xc bike.
You could probably put the longer fork on the STP without voiding the warranty (you'll want to double-check with your local Giant dealer), but that much travel will really slack it out, since it is designed around 80-100mm travel. If you want something a little longer, I'd go with 120mm, maybe 130mm at longest in order to keep decent handling.
