PFK has a review of external filters this month, the fluval has some very good features, but the eheim on test is a more basic one. I would go for size and ease of use. Fluval have served me well for years, but I don't know much about eheim.
i was going to change my external for a eheim but after reading the article in PFK i've decided to go for the fluval - media is easier for me to get hold of then any other as well - i take that into account as well as the filter.
We can only please one person a day, today is not your day and tomorrow doesn't look good either