I own the Chronos II and the Chronos GX. The GX is indeed smaller and has a smaller display. The programming options are also more limited in the GX but it still has all you will ever need for any kind of Chess play. If I were only going to buy one, it would be the Chronos II. The display is larger and shows just a bit more in larger numbers, the delay or increment for instance.
Now, if you want the best at this time so you will not need to worry about upgrading in the future, I would look at the V-Tek 300. This clock is for sale at The Chess House (www.chesshouse.com) and at American Chess Equipment (www.amchesseq.com). The V-Tek 300 has the same build quality as the Chronos, and is even identical in some of the parts. It is programmed to be a lot easier to set and the technology, like the LED indicator lights and displays, is newer compared to the Chronos which is about 15 years old at best.
However, I still own and use 3 Chronos clocks even though I now own 2 V-Tek 300 clocks. In fact I used my black Chronos II this past Monday night at our club for a Blitz event.
If I were to only own one clock it would be the V-Tek 300, because it is well suited for all kinds of Chess play.
I'm new to this but like playing on the app on my phone with a timer and want to get a timer for irl. I am looking at the two Chronos Chess clocks but am not sure what the difference is. I tend to buy top end gear starting out so I don't feel like I need to upgrade later on. Just looking at them the GX seems smaller is that true? Are there functions that the II has the GX doesn't and what are they? Thanks in advance for your help and sorry for my ignorance.