I've now tried out by sentrylogin and gopaywall.
Gopaywall is almost an awesome product. Pros-Can use coupons, it's easy to setup with minimal programming knowledge, customizing is pretty easy, you can use buttons that match your site, can easily do fake transactions to test the system, great user interface
Cons-support is on the slow side, and settings don't seem to behave how you would expect them to. I went round and round trying to get the system to go directly to the locked page immediately after the user makes a purchase, but instead it kept opening an account window, the user would then have to click the page behind the account then click through links to get to the locked page, which was way too confusing, especially since there was no link in the account box to take them to the page they had just purchased access to. Account window doesn't seem customizable to match your site from the member's side of things.
Sentrylogin - Pros-you can totally control where they land after they login by setting the URL, the behavior system makes more sense. Easier integration with stripe (just a couple of clicks). Fast support - response times seem to be within an hour on business days. $4.95/month + 1% fee on stripe transactions, which is super affordable if this is your first time with something like this. Based on their quick support response time and the fact that they are updating their UI, I am guessing they are willing to continue adding features to improve the system.
Cons - more difficult to set up, if you want to customize buttons, etc, maybe they can help but I was working on it late at night so I just did it all myself - so things like centering the button on the page required some research into CSS. I don't see a coupons option, so running sales will take a little more effort. You can't really make buttons match your template buttons, you can't really make the login box match your stuff if you are using certain fonts that they don't offer (like Josefin Sans), you can't edit the purchase page to match your stuff yourself (have to have them create a skin, which they do for free but control freaks with designer work histories may find this annoying). They are partway through updating their 2001 looking website and UI - when they have finished updating I think it will make for a much easier user experience from an admin standpoint. I'm interested to see how this product shapes up.
In all, I suppose you need to decide which elements are more important for you - I think for me, having the system automatically take the customer to the protected content in the least confusing way possible is one of my top needs and I can work around any of the other stuff.
It's also possible that the more expensive mymemberspace is the holy grail of login protection, however, at the price it doesn't make sense unless you have an established, successful membership or e-course program already. If just starting out and not sure how well your product will do, stick with a less expensive option.