Recommendations for Dropbox alternatives?

I use and like Dropbox, but need more storage - about 25GB. I am willing to pay for storage, but the only paid tier that Dropbox offers is $10 per month for 1TB. I don't need 1TB. So I am looking for an alternative to Dropbox that offers 20-50GB of storage for $1-$2 dollars per month, similar to what Apple and Google charge. I know there are lots of cloud storage solutions, but am hoping someone can comment on their experience with a good Dropbox alternative.

In addition to the basic features of cloud storage solutions, I have two "wants":

1. On Mac, I want my files to be accessible via Finder (Dropbox does this by putting a folder in Finder). On iOS, I want my files accessible via an app (not a web browser).

2. I want all my files accessible even when I have no internet access (Dropbox does this by automatically syncing entire folders to each device, so that there is a local copy on each device, which is constantly synced to the Dropbox master library when connected to the internet). Having to manually download folders and files in anticipation that I might want to access them offline is a non-starter for me.

FYI that I am in an Apple-only environment, so only need a solution that works on OS X and iOS. Also FYI that I use iCloud, but find the syncing slow and unreliable.

Thanks!