Hi Natalia,
For somewhat recent information on JSPUI, be sure to look at the slides Cineca presented at OR14 in Helsinki.
It is probably best to just look at all the slides, but they provide their opinion on page 92/93.
Basically, both UI's are supported by a faction of committers and the community. They don't have full parity of features (one might have something the other doesn't), but are pretty close. To an end user or administrator they have similar feature sets.
JSPUI has made some very impressive improvements in the past year or so. It has a lot of administrative buttons, and its Batch Import from the UI works better as it runs as a background thread. But some statistics viewers aren't available in JSPUI.
XMLUI, for developers, relies on cocoon, pipelines, and xslt transformations, which has a pretty steep learning curve. To change a piece of content on a page will involve checking a dozen possible generators, sitemaps, xsl stylesheets, and then you might still be limited, unless you want to tackle a big project. XMLUI seems to have the most currently active user base at the moment. To an end user or administrator it has a lot of features.
JSPUI, for developers, uses JSPs, which mix html and java into the view. It could be better arranged such that less Java logic happened in the views. However, JSPUI is much much simpler to work with than XMLUI.
Not relevant at the current moment, but the DSpace developer community is looking at plans for a "next UI" that picks a best new technology framework to build a new user interface from. One that can hopefully replace both xmlui and jspui, and have users adopt that new UI in the future, and not have the decision to make about which UI, or splitting the user community. There are some prototype UI's built on top of the REST API.