"Best" will depend on your tank size, your filtration, your lighting and your goals/objective, as well as budget. It's not clear regarding the exact contents of the kit you ordered or what your tank dimensions are so it's difficult to suggest a diffuser/reactor.
Themuleous beat me again
By the looks of the picture, you just plug your CO2 line into the bottom connection and position it somewhere low down in the tank, preferably somewhere with good flow. It looks like it will act like one of the glass one's you mentioned earlier.
Good luck, but just be prepared for a lot of tweaking of the delivery rates. Took me a while to get mine right, now I always aim for lime green bordering on yellow on the drop checker. The plants are loving it and no livestock casualties as of yet.
Might be a good idea to set it up when you've got some time on your hands, maybe at the weekend, as you will need to observe the effect your adjustments are having. Your fish will be the best indicator as to whether you've overdone it or not as they will respond to high CO2 (gasping for air), much quicker than the drop checker changes colour.
You could set it up without the DC, but just up the amount you add slowly over a day or so. In the absolute worst case the fish may start gaping at the surface/looking listless, if that happens you know you've got to much. But you shouldn't damage the fish if you do reach that point.