The basic difference between Ancestry and MH appears to be that Ancestry provides vital records,and a search engine to locate them and then you attach these to your tree, The information on MH and its other site, Geni is user-generated, so there can be interesting family information and photos. On the other hand, BMD information and relationships can often be either wishful thinking or very vague, . MH consists of private sites, with a primary owner, while Geni, as someone pointed out, has only a single tree, a collaborative effort.. Geni now has people working hard to iron out the discrepancies which are bound to occur.
Until quite recently, Ancestry concentrated far more on records than on personal trees and I think that that's what they do best. In contrast they give very imprecise information to users of their DNA tests compared to other companies, relying on their large user-base to give them an advantage.
,MH has a lively community of people looking for ancestors in Poland, Russia and Eastern Europe generally. My current experience of Ancestry is confined to the UK, where they provide census records, parish records and directories but only indexes for civil BMD records (cheapest from the General Record Office). but FMP has a much better search engine for British records.
In the past.Ancestry,com have not been helpful for German ancestors, where there is no substitute for Famlysearch microfilms, many now online..
I'd always advise people to keep a personal tree, with images of original records on their own computer, to avoid being tied for ever to one subscription site. Your research may move in an unexpected direction, your needs (and income) may change and in any case it's worth trying several sites to see what they are offering.
Once you've registered, all the sites send you hints and marketing emails,. How useful they are depends on what stage you are in your research.
St Albans, UK.