Q: sharing trough iCloud, or Dropbox ? Both you have to pay

iCloud and its 5 GBs of initial storage are free. Dropbox only provides 2 GBs free. However, iCloud and DropBox are not competing services. They don't both do the same things.

Dropbox is better, if you need to upload many photos that you cannot handle in the phone, because of small disk space ?

iCloud (Shared Photo Stream, journals) will only store your photos, as long as you keep them in your albums or camera roll on your iPhone or on your mac. It will sync with the source, that uploaded to the cloud, and if you delete the photo from the source, it will be removed from the cloud as well. So if you want to remove photos from your phone and store them in to cloud you have to move them to the cloud from a different device, for example from your mac.

Dropbox is behaving differently: it really is a shared cloud storage. Items added to the dropbox will stay, even if you delete them from the device that stored them there.

Regards

Léonie

But the free 5 Gb are not for use with the sharing iPhoto/iCloud feature. Right ? You need to buy extra space for that particular option

Have a look at Photo Stream FAQ

It says:

Does Photo Stream use my iCloud storage?

No. Photos uploaded to My Photo Stream or Shared Photo Streams do not count against your iCloud storage.

This does not mean, that you cannot use iCloud to share your photos using a Shared Photo Stream, it means, that the space required for the photos will not be counted against your free allowance for documents in the iCloud storage. You cannot even buy more space for photos. The Photo Stream will hold only 1000 photos.

That is for iTunes Match which is for music, not photos.

iCloud holds the lessor of 30 days photos or 1000 photos - it is a place to share photos not to store tehm

LN

The 30 days limit is only true for the "My Photo Stream" album, Larry, - all mentions of this period in Apple documents only refer to "My Photo Stream", not to "Shared Photo Streams" and Journals. My web journals and Shared Streams still hold all photos I put there in early September. It would not make much sense to offer tools to layout elaborate web journals with all kinds of widgets, or to create shared photo streams with comments by the the invitees and to publish these streams on the web, when  all photos will be gone after 30 days.

see: iCloud: Photo Stream FAQ

How long are My Photo Stream photos stored in iCloud?

The photos you upload to My Photo Stream are stored in iCloud for 30 days to give your devices plenty of time to connect and download them

But the shared photo stream documentation does not mention any limited period of storage at all:

iCloud: Using and troubleshooting Shared Photo Streams

The same holds for Journals published on the web: iPhoto for iOS (iPad): Create a journal

With "Shared Photo Streams" and "Journals" published on the web has Apple done a step back towards cloud based web publishing, but without openly advertizing this fact.