A500+ and A600 are essential the same machines. The 600 does not have a full size keyboard as the 500+, yet with the 600 you will get 16-bit PCMCIA port and 44-pin HDD connector. Just be aware that out of the three kickroms ever released by commodore, the first version had no HDD-Functions.
You can identify them visually, by the sticker that say eighter A600-HD (eighter 20 or 40 written before or after "HD").
If the "HD" is missing, you can allways see the kickrom version, wich is written on the screen after a reboot (The one with a Floppy, telling you to insert one).
I would go for A600 or A1200. Only be aware of the clockport bug on the 1200, wich can be solved by some soldering work (can't remember exactly how/what), wich is documented throughout the internet. The advantage of the 1200 is that you get a hell of upgrading possibilities. Wich is only surpased by the 4000 model. With the 600 and the 1200, you can even load a kickrom changer disc. Kind of like a softmod. It need's to be used with the settings, found in the Amiga equivelant of the BIOS. You enter it, by booting the 600 or 1200 while holding down both buttons on the mouse.
Then you can set it for eighter OCS, ECS or AGA on the 1200 and OCS or ECS on the 600. The main disadvantage of the 600 is the lack of upgrade possibilities, as more was invented for the 1200 and even the 500.
Hope you can use these advices.
EDIT: (Forgot to add)
With the 600 and the 1200, you have this 44pin HDD adaptor inside and the PCMCIA port. With this you can use a simple 44pin CF-to-IDE adaptor, and read/write the filesystem on a Linux machine. You can even mount an Amiga HDD inside UAE on whatever host OS you like.
The PCMCIA port can be used for an extra drive, with the use of an 16-bit PCMCIA to CF adaptor, or the port can be used to drive a "SlowRAM" PCMCIA card, or even to be used with an SCSI adaptor if you can find one. I had one back in 2006, that was named something like "Squirrel". That I used with an external Plextor SCSI CDROM burner.
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Those cakes make you sick....
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