The answers you will get will probably be very subjective to the users likes or dislikes of a product, the being said you will need to factor in several different things when choosing your tool, here are a few off the top of my head.
1. The type of application you will be testing (web Siebel, Power Builder Java, etc...)
2. The availability of resources for the product you will be using (new hires)
That being said I like the interface that Segue offers for Silk Performer (I must admit I have not seen Load Runner in a while) and I was very disappointed with the Rational tool (I have very limited experience with it but it seemed very code intensive).
Please feel free to contact me if you have any other questions.
But in the end doesn't it all come down to BEER? Beer is the ultimate answer to all questions in the universe so yes the answer to your question is BEER.
Each tool has its pros and cons. Check out the "favorite tools" threads in the performance testing forum.
Some important factors you should consider when selecting a tool (and which don't often get mentioned) are personnel and training. How hard will it be to find job candidates in your area with skills in the tool you choose? Is there training available where your company is based, or will you have to send staff out of town for one or more weeks? If you are able, I'd also suggest taking a look at the message boards and knowledge bases for the products you're considering to see how much information is available online from the company and other users.
Both have pros and cons. Performer seems to have an edge on test analysis and handling script parsing in an easy manner. LoadRunner has some good test data management features that make some aspects easier. LoadRunner costs a lot and add-on features are licensed on the VU's which makes getting an add-on a LOT more expensive than SilkPerformer. Both have similar learning curves.