Hard to compare unless your library is rather small.
With a 300 song library as an example, you can either spend $300 on iTunes & have that library for use whenever you want without any sort of restriction/limit. Or you can eventually spend $300 on Spotify & had access to those songs for 2.5 years.
In the long run, the iTunes route will be cheaper since you're not "renting" the songs, but at the same time, the bigger your playlist gets on Spotify, the more time it will take for the money you spend on it to equal the money you'd have to spend on iTunes to have all those songs available immediately.
In an extreme case, you could spend $1,000 on iTunes for a 1,000 songs, but by the time you spend $1,000 with Spotify, you would have had a subscription for over what? 7-8 years? And in such a time frame, you would have realistically kept adding even more songs with Spotify or spending more money with iTunes.
Personally, I've had Spotify for 7-8 months now & it hasn't bothered me to have spent $70 for over 400 songs in those months instead of just spending $400 on iTunes at once. The only thing that bothers me about Spotify is that some cases, songs disappear from their catalog without warning, but it appears rather rare.
The other downside is that there are popular artists that have not allowed Spotify to use their music. iTunes has this issue, but it's not to the same degree. However, in those cases, you can easily buy for example, Metallica or Pink Floyd albums, & merge them with your Spotify playlists with it recognizing those are your songs.
Ultimately, it comes down to whether or not you want full ownership & you have the wallet to support it.
