A600 = 68000@7Mhz, 1Mb chipmem, ECS Graphics, Kickstart2.05, Missing keypad, PCMCIA port, Only expansion slot for extra 1Mb chipmemory... other uppgrades are reversesockled onto chips on the motherboard.
A1200 = 68020@14Mhz, 2Mb chipmem, AGA Graphics, Kickstart3.0/3.1, Full keyboard, PCMCIA port (missing reset), Clockport expansion and CPUslot expansion with possibility for PPC & PCI/Zorro bussboards... other uppgrades are reversesockled onto chips on the motherboard.
It doesn't take a genious to realise that the A1200 is a wastly better machine than the A600. That said I wouldn't want to let go of my A600 as it's such a cute little machine. Also with the right uppgrades it's a great machine for putting in a backpack to take with you on a party... just as long as you got something more powerfull at home first.
So get an A1200 first... and if you then need a lighter and smaller travel partner then get the A600 later on also.