I've got the 6-24x, it's ok for the price tag but not a show stopper, how can it be.
It gets milky at higher mag and the clarity seriously drops off towards the other edges.
You get what you pay for.
As far as which model, as Mike said it depends on application and personal preference.
Some people are happy shooting at 6 -10x mag even at long distance, while I prefer higher mag even up close.
All I know is that Gamo glass is of consistent quality, the mechanics are sound and the local support is there. I have owned quite a few and put them through hell and back. I have never looked through a Discovery, but the price tells me they are 'probably' junk when compared. You may luck out and get a gem, but that is always the case with cheaper brands... their QC sucks.
Perhaps go for the 16X Discovery. It's a good mag for longer distance and likely to be clearer than the 24X.
Yes I am very happy with it as I said, on my CP! and shoot out to 20 meters in my backyard. I do not know what they would be like on Springers, but PCP and CO2 they are fine. They used to be called Macool but have since been rebranded Discovery. Anything further than 25 meters and then I change rifles and scope accordingly.
The most important thing is SUPPORT for the product.
Perhaps someone could elucidate us on how it stands for these two products nowadays? History on one brand was not so great in the past.
Having said that, IMO, the 16X Targetmaster is the PERFECT utility scope. Big enough to shoot long distance with a nice fine reticle and side focus.