This is a no brainer for me, let's pretend I'm your SEO and I have just found out that you are using sitemaps.
Is it true that the big 4 search engines are all link based, this means that for a page to do well in a Search Engine it needs 2 things .. internal links and external links ?
So that in mind if a Search Engine can't find a page on your site, would it not be better to go get a link or two and point it to that page, that way we know the SE has found your site by a method which it values !
I have seen too often recently, people using XML sitemaps to get pages indexed and then they just never rank, then they ask me why these pages don't rank..
So I have to say well what pages did the search engine find to give relevancy too, if the answer is, "I don't we use a XML sitemaps", then how the hell do you expect me to improve what pages rank if we don't know what pages the SE found the natural way.
In the Search Engines eyes it looks like a standalone page, an island, a doorway page. Bad internal linkage should never be a reason to use an XML sitemap, if you have indexing problems don't make it worse by hiding the problem.
I would rather have 10 pages indexed from a 1000 page site than 1000 pages included in a index because of an XML sitemap, IMO XML sitemaps are a quick route to supplemental hell. Make your SEO's life easier and don't use an XML sitemap.