Since you already have Ancestry and My Heritage I would say hold off on anything else at the minute until you have exhausted all other resources.
Family Search, FreeBMD and other free sites will have all sorts of records you can use.
Google can be quite useful especially if you have a specific area and names. For example, if you are looking for family that came from a village in Wales use Google to search and you might find local histories & history groups, genealogies, etc. (search name + place but keep changing search terms to bring up different results).
Here on Rootschat go to the particular county board of your interest and check out the resources section for places to look. Then, still on that board, take a look at other queries as often replies can point you to other sources also,
Use maps to learn about an area- sometimes an area is known by more than one name and mislead you into thinking relatives moved house. Google Streetview, when available, can give you an idea where your family lived and what it looks like now.
Newspapers are a great resource but coverage isn't always for the area and period you want so check availability on sites.
Then, after you've done all the above... have a look at what other paid sites offer and think about the gaps you want to fill in. For example, if you family came from Bangor, North Wales and only FindMyPast has the church records online that would be something useful to consider.
I think most pay sites might offer a free trial which is a good way of seeing the layout- a good search engine that is easy to use is very helpful.