Ok, first of all, very few people have worked for both companies so you're probably not going to get anyone who can give you any true insight into hills vs RC as a company to work for.
As far as product lines go and what (I would imagine) obstacles you will face on the sales end:
As a Hill's rep, you are the industry standard. Nearly every practice will sell your product. Not really much innovative with your product line. Market share is declining for you as RC steals business and clients generally are moving away from prescription pet foods. You're largest challenge is simply to keep from losing business.
As an RC Rep, you have the more expensive product and while innovative, many doctors don't believe the hype with many of the products. You're biggest challenge is convincing Ictors your more expensive product is actually superior and getting them to change how they've been dealing with pet food their whole career.
Listen, most vets don't sell. They didn't get into animal health to sell to clients so many simpy aren't very good at it. So the job (regardless of company) is frustrating and not as easy to "move the needle" as you might think. You are relying on these doctors to sell your product for you and many don't give a shit about pet food, especially those that have no vested interest in the bottom line for the clinic.
So, good luck in whatever you decide. I feel you add what I've said here along with the micro management style at RC and you get people being vocal on these boards. Maybe Hills is better on the company end but the job is still as shitty, I assure you.