Guess this is a good time to post my old information about this subject.
Old post about subscriptions
As for subscriptions, if it only reminds someone to donate, then its not really much of a subscription.
I'd say both. Use 1 or 3 depending on needs.
For system 1, It would be easy because you would just have paypal build the subscriptions and you would copy/paste the code in the subscription area for the different values.
For system 3, I would have a subscription base much like paypal's but its a subscription they can enter in the amount and have it auto-deduct much like paypals. Paypal you need to choose the amount in advanced.