If you really love them, I'd keep those you love the most or the ones that would cost more to replace, as you can always buy another of the lesser-worth ones for about what you sold them for, if you ever have the cash to get them again.

If you are seriously on thin ice with cash, I'd sell what you feel comfortable selling but maybe let go of a few to good homes.

IMO, it's what you feel is right and what you really think is necessary for you to have and how much you'd need to get rid of, if it means a suitable amount of money.

I'm not the best to ask about this and I'm sure other will be able to help much better than I can, but like you said, it's a hard decision on what and what not.