I have not tried the Silver Line Express bus yet but have tried the other two options to IAD and then used the Washington Flyer Bus when it operated. Here are my takes on the different options:
5A- I tried hard to think about why would anyone want to take it over the other options. I came up with they are traveling on the green line and the bus was there and the other two were not. It is the least reliable, most expensive and least comfortable public transportation option to/from IAD. As long as I live, I will refuse to take the 5A. Way too many variables that could make the trip miserable( traffic in DC, 395, Rosslyn, 66, crowded bus that you never knew your body could move like that etc).
Fairfax Connector 981/983- Knock on wood but maybe I have been lucky traveling with Fairfax Connector buses. They have been reliable for the most part. Seats are fine and it goes to places that I need to. The buses take a while to get from Wiehle to/from Dulles but a very good option. The 3 times I have taken it, there have been maybe 5 people on the bus combined with me. I think people need to promote it a lot more than the 5A and Silver Line Express bus. Seats are very comfortable, while no rack to put luggage in still plenty of space to put them on the seats etc.
Silver Line Express- The Bus stop for the Silver Line Express is right next to the Fairfax Connector 981/983 bus at Wiehle. Regardless of when you arrive at the station, there seems be to be always a Silver Line bus. For the guy who said there was bad signage, I could have sworn there was signage for the Silver Line Express when you leave the station. Regardless, if it is anything like the Washington Flyer Bus was it was extremely reliable, very relaxing etc.
I will take going to Wiehle Station 100 times out of 100 even on weekends when they are running every 15+ minutes. It is a lot more relaxing being at the first stop on the line than getting limited space on the 5A bus.
Let's look at worst case scenario: If somehow the Silver Line and 981/983 buses are not there at Wiehle, you can always take a fairly cheap cab ride to IAD. When there are problems with the 5A , you pretty much screwed if you are on it. You can go back down the metro to Wiehle but it could have saved you an extra 20+ minutes with staying on the metro out to Wiehle.