Probably fin type heater core as it is very similar to a regular auto radiator in design and that type is chosen for a very good reason. I doubt you'll see any/many tubular radiators in an automobile - the bore of the pipe to maintain flow would too large to be an efficient heat exchanger.
The automotive type radiator heater cores have much larger surface areas for the water - means the heat exchange is much more efficent. It is highly recommended that you use a heater core over a transmission type cooler.
I have had a chance to use both types ,a good copper tube and aluminum fin rad works almost as well as a heater core , the one difference is price ,I paid 35$ for a used tube rad. its nice and all ,but for less than 10$ I can pick through hundreds of heater cores at the local junk yard in aluminum as well as brass/copper and all of them are as good or better than anything available online
You have to remember that you're not trying to dissipate large amounts of heat here, so either type of radiator will in fact probably do the job required. Its' only when you are reducing the size of the system does it really start to impact it's ability to get rid of the heat.
There are so many factors such as flow rate, surface area (both radiator and waterblock), air flow volume through the radiator, ambient temp, etc etc. If you want to miniaturize your cooling system then efficiency becomes more of an issue. There is so much headroom or "overkill" in a water cooling system that it will usually only mean a few degrees difference in the final temps for fin vs tube - so price is much more of an issue.
ReNeG@dE - Heater cores probably all have similar sized inlet/outlet tubes. Block one end, put a tube on the other, use clamps if they are a little loose. Connect a bicycle pump to the tube, put the rad in a bucket of water and then pressurize the rad - if ya don't see any bubbles it's a keeper. Of course, it may be full of crud and will start to leak after you clean it.
I also found that if you go to the local auto-parts store and ask to look at the application book, you will find a picture section that has drawings and specs.
some of the new ones don't cost that much $15-$30 in sizes from 5 1/4 - 5 1/4 to 6 1/2 - 6 1/2 and 2 - 3in.
thick, also with different inlets and outlets and maybe get warranty.