We did both when we went to NZ in Dec. 2008. You can read about them on our blog:
http://www.wcthompson.net/journal/?page_id=550, follow the links and you will see each track broken down into days, and plenty of photos and videos.
My personal opinion was that the Routeburn track was stunning, and that the Kepler was very plain, don't give the Kepler more than 2 nights, if you are fit enough. I would err towards giving more time to the Routeburn, stay at the Lake Howden hut (1.5hrs downhill from it to the Routeburn Divide), and skip Te Anau (which is great, but not really).
The Routeburn track was the first track that we ever did. The first day was gruelling for us, but by the time we hit the hollyford we had found our groove. We also stayed in the less frequented huts (ie Routeburn Flats Hut rather than Routeburn Falls Hut, which you would never do if you weren't unfit like us, but it was a great decision for us - and it was so much more peaceful). However on the 4th day, we left Lake Howden Hut and went straight to our next track (the Hollyford), we even decided to skip the hut that we were supposed to stay at and keep walking to the better hut, making for a 23km day with stupidly heavy packs - I say this only as agreement, that going from the end of the Routeburn onto the Kepler shouldn't be a problem. - The sun doesn't set until 8.30pm in Summer, and it is still light for quite a while afterwards.
The final day of the Routeburn is relatively downhill (the last hour is practically all downhill, we jogged with our 15kg+ packs), the first leg of the Kepler is unrelenting uphill for 3/4 of the trek, but the path is awesome - you could almost take a pram on it. It is also hugely popular, I counted at least 60 people, day walkers etc. However after that it is very easy going.
Other things to note:
Both the Routeburn Flats Hut and the Lake Howden Hut were quiet (one other person, plus a warden), as opposed to the two main huts which house 40 people and are absolutely packed.
The cave at Luxmore hut is fun, worth a stop for the experience of caving in a relatively safe cave (it is tight and scary, but has only one track, and plenty of people are doing it).