I've been a hardcore in-tank user for a long-time. Didn't matter if the tank was 3 Gallons or 75 Gallons. I always used in-tank. The ADA way. I did recently switch one of my tanks over to an inline one and am very happy with it. I would say for optimal performance you probably need to clean an in-tank one every 2-3 weeks, but YMMV. Cleaning is a simple 1:1 water/bleach dip. I haven't cleaned my inline one since I started using it 4 months ago. No light exposure, no algae.
They both have adv/disad. In-tank allows you to see immediately that the co2 is working. Many diffusers also allow you to see the bubble count as well. There is some thought that the co2 bubbles can have a positive effect on plant growth, but both methods work so i'm not sure how much of a real factor that is. The disadvantage of in-tank is obvious since they require the regular cleaning and you are also placing another piece of equipment in the tank. Some of the inline diffusers as actually atomizers and they usually require a higher working pressure than the in-tank diffusers.