I prefer external reactors for a few reasons. One is 100% dissolved CO2. Everything is dissolved, so even though I get my CO2 for free, I use less. Second is no bubbles in the aquarium. I just don't like it. The water may be 100% clear, but with all the fine bubbles it looks a little cloudy IMO. Third, nothing visible in the aquarium, so it's all about the scape.
That being said, a CO2 (glass or not) diffusor has one big advantage as well. All the microbubbles in the water have one plus point, small particles that cloud the water stick to the small CO2 bubbles. This way they make larger particles and the larger particles are easier to trap for the filter. So in the end the water will get 'more clear' (less suspended micro particles). You pick the advantages you like.