Internal zoom/focus pros:
- Easier to fully seal up.
- Easier to optimise focus motors for speed.
- Usually results in better balance.
External zoom/focus pros:
- Usually cheaper to design and produce, which is a saving often passed on to the consumer.
- Usually lighter.
- Though often longer at maximum length, can often be much shorter at minimum length.
- Easier to optimise focus motors for precision.
- Easier to avoid focus breathing and get closer to the stated focal lengths (e.g. '200mm' is often something like 185mm with internal zoom, but might be 198mm with external zoom) .
- Usually better optical quality at minimum and maximum focus distance.
For
most regular uses, external zoom is the better option. People seem to often overestimate how much sealing is required for standard use in a typical city or town (or even for travel) and focus speed of modern lenses is also often overkill for the purposes of the average working professional. To put it simply, unless you're a wildlife photographer trying to hunt down birds of paradise in the jungles of Papua New Guinea, you're unlikely to need as much sealing and AF speed as you think.
After all, people have lenses which are decades old without any dust or moisture leaks in them, and for most purposes getting accurate shots is more important than fast shots; speed means nothing if your shot is out of focus.
The average wedding photographer, portrait photographer (studio or location), product photographer, and all but the most extreme landscape, sports, and wildlife photographers, would be better off with external zooming lenses (and external focusing!) where you can more easily and cheaply get the best possible optical quality and you can save some space and weight in your camera bag.
I'm reminded of a friend of mine who asked me what lenses I would recommend he bought for general day-to-day shooting, and a little bit of travel, with his new Fuji system. I suggested the little and cheap 18-55 f/2.8-4 OIS kit lens, for stability and size, and a 23mm or 35mm f/1.4 for when he really wanted more light than f/4 would allow and OIS wasn't enough. (Bearing in mind the Fuji system is APS-C, so those are 35mm and 50mm equivalents.) He disregarded this advice and insisted he needed weather sealing, so bought the expensive and large 16-55 f/2.8 instead. Three months later he sold the 16-55 and bought the smaller lenses.
Internal zoom and focus looks slicker and more high-tech, but from my experience with both my own work, a wide variety of pros, and contacts within the manufacturing and sales side of the industry, I'd say 9/10 pros and 19/20 amateurs would be better off with external zoom and focus. Very few people need that higher degree of sealing and AF speed, while many, many people would benefit a lot from the savings in weight, price, and consistency in optical quality that external zoom and focus allow.
To put it really simply, if the RF 70-200 zooms and focuses externally then it has a better chance of being as optically good as possible.
Since the R line isn't yet ready for heavy-duty extreme weather like the 1D or 7D cameras are, nor are they ready for the extreme speed demands of professional sports and wildlife, I don't think the lower degree of weather sealing and potentially slower AF matters. Saving weight, keeping the minimum size low, and maximising optical quality are far more important for this particular lens for this particular system as it currently is. Maybe in 5-10 years we'll want a slicker lens to match whatever R bodies are available then and we'll get an internally zooming/focusing 70-200 to match, but for now, external is definitely the smarter way to go.