No! Both do a good job at a reasonable price.
I have used both and they are both very good services.. also both drop you close to the Impala round about.
There's more then that, but these two are the most often used.
They use small 24 passenger buses, and my only comment would be to make certain that you're not over the rear wheels (you will be exhausted by the time you get to Arusha if you are and you will feel every bump in the road).
I have used larger buses sometimes in the past and they are much more comfortable for a ride of that distance, some with restrooms onboard. One which does this has a web site www.aashuttles.com. They're all about the same price.
We took the Imapala shuttle near the end of May and it was fine. A friend of mine reminded me that not long ago Riverside lost bus enroute (accident) while Impala has fared better.
Due to the road work, be prepared for a 6 hour trip, a lot of dust and a VERY warm ride. We only had 8 of us on the bus and it was already pretty crowded feeling. I'd hate to ride them when they are FULL.
You could use either of the bus services, they are not too bad. Keep in mind these are normal buses and not luxury nor are they very comfortable ; but a 5 hour ride with a break halfway at the border is not too bad.
I made the trip once from Nairobi to Arusha on Riverside and they put me in what they called a centre seat that opens out onto the aisle. The part that got me mad was I had actually reserved seats but since I was the last person to get on the bus ( got picked up enroute at the airport ) they gave my seat away. I was furious but guess what there was a Policeman in plain clothes travelling on my seat, how could I argue !
I know with Impala the system is well organised and if you book a seat number you will get the same one ( You can choose on their chart ). I had family who flew in from Zimb to Nairobi and made a return trip to Arusha with the Impala Shuttle and had a good trip.
Since there is still construction happening on the Nairobi-Arusha road, you might be better advised to not take the small bus (24 passengers) and find one of the larger buses. They take the bumps much better and the ride is much more comfortable with the larger wheel-based 60 passenger buses. You might want to look into www.aashuttles.com and others such as Scandinavia Bus Line. There are others which are plying this route going from Nairobi to Dar es Salaam, which causes them to pass through Arusha.
I've had experiences with both and the 60 passenger bus was a piece of cake compared to the smaller buses operated by Riverside and Impala.
Cost is about the same, comfort level is much different, especially on the roads under construction.
Karl makes a good point about comfort. I just want to add that some of these larger buses, I presume, require a visit to the Nairobi Bus Stand which should never been taken lightly.
I believe Scandinavian Bus will probably not require a visit to the bus stand, but definitely ask, as the bus stand is infamous for muggings.
I agree, as I would agree with most of the bus stands in E. Africa (such as Ubungo Bus Stand in Tz).
I believe that some of the shuttle kinds of buses, including the 60 passenger buses, pick up at the city center bus stand near the park (I think it's called Parkside hotel). One of the 60 passenger buses which I've taken in the past is called (I won't have this right) something like Donovao (SP?).
Just a quick update. Scandanavia has closed down. Dont know if its temporary or permanent but its not operation at the moment.