BMW R1250RT versus the BMWR1250GS Adventure?

So you think you want to buy a BMW Bike?

It's a great choice, whichever model you purchase. BMW make extraordinary motorcycles with the finest technology, at a very high quality. Fact. Their software came right of the BMW World Superbikes of a few years back. If you are used to Harley Davidson by comparison, you will be astounded by the quality improvements on the BMW. Its that simple.  You will also be very aware of the lighter weight too. I will give you some examples below.

GENERAL COMMENTS

1. The engine is essentially the same. The "Boxer" two cyclinder works because the heads are cooled in the breeze. They also stop the bike falling "Right over" if you drop it, making it easier to bring up-right. Over the years (since my 1985 R800RT) they have made many small but significant changes to the boxer such as liquid cooled and dual spark plugs. (BTW, the "R" series has the boxer engine, the "F" series BMW bikes are parallel twins, and the "K" series are the inline sixes.)
2. Both bikes use Shaft Drive which makes for easy maintenance. 
3. The suspension on both bikes is particularly fine. It is longer on the GSA, but both will surprise you. Read up on the "Duolever" and "Paralever".
4. The new ten-inch  TFT screen on both bikes is exceptional.
5. Both bikes have a side stand and centre stand, which makes wheel changes a breeze.
6. The horsepower is similar at 134 HP on the GSA (136 on the RT), and top speed is "well in excess" of 200km/hour - you dont need more. These figures make HD look slow, if you want to use the top-end.
7. Both have adjustable ride modes like "sport" or "rain" and I use them. 
8. They both have the same quick-shifter, same shaft drive, same engine etc, so there are many things that are identical. I should note that the engine gets better and better the more it is ridden to about 40,000 km. Many people have kept their bikes well past 400,000 km. My 1985 R80RT was sold when it had 283,000 km on the clock and was running perfectly. 

THE "RT" bonuses

1. The RT is a sealed road, highway-and-corners bike. While you can take it on the gravel, its not made for that. Cleaning it would be real work!
2. Protection from the weather is unbelievable. 
3. The RT is incredibly comfortable with both an adjustable screen and adjustable seat (which can be heated if you want). The standard seat needs no alteration.
4. The ride position is very neutral and balanced. My knees are at 90 degress with the seat in the middle or high position. You can buy bar-risers from Touratech or others if you want to lift the handlebars, but I doubt you will need it.
5. The RT has central locking for the controls and the panniers. This is very convenient. The GSA does not - you need to individually lock those huge metal panniers or top-box.
6. The RT is a sports tourer. It's handling is exceptionally good. It's overall highway and corners handling will leave almost all other bikes in their wake. The "rake and trail" for this bike is made for corners and highways on sealed roads.
7. The bike comes with cruise control. I happen to think that the BMW Cruise control is by far the easiest to use. But more importantly, for the RT has "adaptive cruise" making the highway ride quite special, as the bike slows for traffic in front.
8. The lights on both bikes are LEDs of very high quality and considerable brightness, but the patterns are different, and very identifiable when coming towards you. 
9. The front wheel is a 17 inch on the RT, vs a 19 on the GSA. The rear tyre is wider on the RT. 
10. Tank size is 25 litres which is plenty for well over 400km of range. (vs 30 on the GSA)

The GS Adventure or "GSA"

1. The BMW R1250GS Adventure (GSA) is the most popular BMW bike bar-none. The reason is probably that Charlie Boreman movie! They say "if you stand on any corner in the world from Ushaia to Vladivostock, one will eventually drive past you" - and I can believe that. 
2. The GSA is, in  my opinion, the best "all round bike" in the world. It is the "Swiss Army Knife" of motorcycles, and therein lies the negatives - it is not perfect at any one thing. It is NOT a dirt bike (too big and too heavy) but its also NOT a dedicated highway machine, even all though it works brilliantly at the latter. 
3. As for the RT, the seat is very special, but different.
4. The suspension has huge travel and that it nice. Its also very adjustable. 
5. The technology on both bikes comes straight off the BMW race bikes like lean-angle-traction control and ABS. 
6. The riding position is taller. It is also very comfortable, but it makes it more of a challenge to get onto the bike, especially if you are shorter.
7. The screen is adjustable, but not electronically like the RT. I also changed my screen to a larger one.
8. The TFT dash is the same. 

Conclusion

I have owned three GSAs (2007, 2014 and 2016) and two RT's (R800RT and R1150RT) and loved them all over 21 years. The technology has advanced so much, it is time to upgrade. 

But the decision is simple, if you want the best highway bike then the R1250RT is unbeatable. Frankly I would buy it over the GSA then own a smaller (non BMW Bike) for the dirt trails.