Capacity wise, they are rated for the same size tanks. If you're running it inside of your tank, then the 9004 actually has a smaller footprint than the Slim Skim so it will be less obtrusive in your DT. If the Hydor Slim Skim standard works similar to it's nano version, than it'll be a pretty poor performer. The Tunze 9004 on the other hand is quite an efficient little skimmer, and Tunze is well known for their quality products and customer service. Either way, depending on how heavily you're stocked, I think either choice will be somewhat underpowered in a 65 gallon tank but you are unfortunately limited in choices as you will be running your skimmer internally.
Do you run an ATO? This will be very important as a constant water level will be vital if you want constant skimmer performance.