I've used both and will share my experience. Both products work well and do a good job. I have only been playing around with Gliss and recently applied it to my brothers Tahoe. He texted me the other day that the beading and sheeting is great and that it is easier to wash and dry it.
In comparing the two products side by side. Reload is slick as soon as it is applied. Gliss from personal experience gets slicker after a few hours to a day later after initial application. Gliss retains the 110 degree contact angle with water where as Reload is around 90 degrees. Meaning Gliss has the edge in hydrophobic properties. The other benefit is the slickness will be longer with Gliss compared to Reload. Reload on the other hand is so easy to use that a fresh application can be used at any time if that is the product chosen to maintain cquartz.
So it comes down to personal preference.
To answer you other question about using Reload over Gliss, again that is personal preference. It will be fine to use them together.
I gave my brother a bottle with Ech20 mixed at 1:20 to use as a drying aid on his Tahoe as it won't alter the hydrophobic properties that much as my personal preference is that of Gliss.