I'd personally rather have the 640 as opposed to the 60, and they are both fine guns. I have both and I own the first 640-1 I ever saw in a gun case. That happened within the past year, and I've been looking in gun cases and on tables at gun shows for a great many years. I prefer the centennial style J frames over the ones with hammers, and I much prefer the longer length barrels as opposed to the 1 7/8" found on many J Frame models.
I'd ask you this question, how many 640's have you had the opportunity to buy? Have you asked if the LGS will accept a slightly lower offer? Many times cash money speaks! I've got a 640 now, and it would be very hard for me to walk away from another. That's a choice you have to make, and you MIGHT find another somewhere. I have noticed that they are not getting any cheaper as time goes by. So what's it worth to you? Doesn't matter what it's worth to any of us. If it suits you and meets your needs or wants, I say it's the best investment you are likely to be able to make. You can make a good investment and be able to enjoy it by shooting it anytime you wish!!