I like the XLAB top-tube bags. They are lightweight, aerodynamic, and provide easy access. No more stopping, getting off my bike, unzipping my under-the-saddle bag, rooting around and hoping nothing falls out. I got the Stealth Pocket 100 for my race day bike. I tried the Stealth Pocket 400c for my training bikes but found it too narrow to hold my training day needs. I returned it and got the Rocket Pocket XL. It has room to hold my emergency tire stuff, my flip phone, hydration packs, with room to spare! And it sits below the top of my headsets for good aerodynamics!
I got this bottle to reduce downtube drag on race day. I lost 4-6 ounces in volume, gained a few grams in weight, but flew down the hills. I have a 54cm S-3 Cervelo and this bottle wouldn't work without adjustment bar. The Wolf Tooth Components B-RAD Mounting Base worked for me. The XLAB Aero Cage Optimizer Versatile Mount was too big for the tight space and weighed twice as much. I got the white one to be able to monitor inside cleanliness. This bottle is expensive, but worth it if you want to reduce downtube water bottle drag.
I bought the Stealth Pocket for my top tube to use in addition to a larger seat post back that holds my spare tube and other repair items. I use the Stealth Pocket for more frequently used items such as gels, chews, tablets, etc. that I can even access while riding or quickly grab during a brief stop. The bag is narrow which is aerodynamic but that does limit its capacity so you have to make some tradeoffs in what and how many items you can pack.
