Although this is an old thread, I found it in the search results when I searched for backlink tool comparison info,
and I think it is worth it to get it back from the dead for new information.
I tried the free version of Seo Spyglass in the past (about 3 years from now) and I found out that it is a very nice tool, full of useful information on backlinks like the penalty risk calculation and the backlink authority finder,
but I found it poor in backlink discovery, since it brought up very few backlinks comparing to other tools.
On the other hand, Ahrefs is an amazing backlink discovery tool (for some of my websites it will find almost all google links in my webmaster console dashboard, and in some cases it finds links that the console will not show).
The bad thing with Ahrefs is it's price, which is very high for a guy that doesn't earn his living from SEO, so I bought the monthly subscription only when I couldn't do different and always paid for a single month.
This week I did a re-researched on the backlink tools available in the market and I fell on Seo Spyglass again.
I downloaded the free version and I found out that the software has improved a lot, and that now it returns a lot more backlinks that it used to bring in the past.
I searched a little on the internet and found out that the company behind Seo Spyglass has released an online backlink tool (webmeup - dot - com) with monthly subscription programs, and a free version, and that Seo Spyglass is integrated with the online database of webmeup (which propably is the reason behind the huge improvement in backlink finding of Seo Spyglass).
So I am thinking of buying the proffessional licence of Seo Spyglass, but I would surely like to hear another opinion before doing so.