If I could easily pick up that 2nd drive cable here in the UK I would go for the mid model as well. Although I can't find it easily in the UK unfortunately so may just go with the server - replace one of the drives with a SSD, and stick the 1TB drive in a USB3 case to use as extra storage.
I've got an old "White mini" server, but am always watching for what I would be moving to next. Back then the choice was obvious -- it was like buying the server software and getting the RAM (4GB rather than 2GB) and second hard disk drive free.
Now the obvious (?) choice is the mid model. $200 for the extra drive and a $20 software package is too much, especially since adding the drive add heat which decreases the reliability. Actually, with the quad core base model, I'd probably just buy that one, perhaps replace the 500GB drive with a 750GB 7200RPM drive, and be really ahead of the game. I've never hit 100% CPU with my dual core server anyway, so don't need the processor speed.