For me the answer is both! Not only do you end up selling more but you have a much more enjoyable time doing so!
When I am coaching corporate sales people and business owners to increase sales with their existing clients I often come across one of two mistakes which get in the way of sales.
Firstly, the sales person only ever talks to the client when they want to sell something. For some reason the sales person gets a little puzzled as to why the client never seems to take their call unless they want to buy something.
The other mistake is that the sales person works very hard on the relationship and never mentions sales unless the client brings it up. The sales person gets very frustrated that despite all their effort in building the relationship they are not selling much.
I would suggest that we should be seeking a happy medium between the two. We need to have both relationship conversations and sales conversations and it is important not to mix the two. The client needs to understand that you are there to add value and that you will be coming to them from time to time with suggestions and new ideas that you believe will add even more value.
One of the key things to remember is that if you are engaged in a relationship conversation and a sales opportunity comes up, that you arrange a separate meeting to discuss the opportunity. In my experience a client is more likely to take your calls when you have demonstrated by your actions that you are a valuable resource to have around and not just a typical sales person. Ironically, the quality of the relationship will increase significantly and so will your sales!