There are many popular records that you can find on more than one genealogy site, and if you don't find what you want one one site, it's a good idea to try another one. There are two different reasons for this. Some records, like most British censuses, have been indexed independently by different service providers; in other cases exactly the same database appears on more than one site, but different search engines mean that one site may give better results than another. This is why it is sometimes worth using a commercial site to search indexes that are free elsewhere.
The Civil Registration birth, marriage and death indexes for England and Wales are available on a number of sites; none of them, sadly, provided by the General Register Office itself. The most widely used is FreeBMD an astonishing example of volunteer effort on a massive scale. As the name implies, it is free to use, and because it has only one function, searching the BMD indexes, it is expressly designed for that purpose, and it works extremely well.
Birth marriage and death indexes are also on Ancestry, Findmypast, TheGenealogist, BMDindexes and FamilyRelatives. BMDindexes is a standalone site which is also part of TheGenealogist, so the indexes are the same, but I can't comment beyond that because I am not a subscriber to either. Ancestry's indexes up to 1915 are the FreeBMD indexes, and are clearly listed as such, but for 1916 to 1983 they have their own indexes. Findmypast and FamilyRelatives have each done their own indexing, so for any period up to 1983 you have a choice of several independently compiled indexes. Indexes from 1984 to 2005 were 'born digital' and the databases used to be sold by the GRO to commercial companies. No indexes more recent than this are available online, with the exception of 2006 births and marriages which are on Findmypast only.
I may look at the relative merits of the various indexes some other time, but what I set out to discuss here was the reasons you might choose to one site rather than another for the same database. Specifically, why would you search for BMDs on Ancestry rather than FreeBMD? I have to admit that I nearly always use FreeBMD. The FreeBMD updates don't reach Ancestry straight away, although this is a minor issue, since most of the activity on FreeBMD these days is in the post-1915 period that they don't share with Ancestry. The question of coverage is still relevant, though, since even for the earlier years FreeBMD is not complete. The FreeBMD site itself has very informative coverage charts, which show you where the gaps are, but you won't find this information on Ancestry.
Another advantage of FreeBMD is that you can search just births, marriages or deaths, or across all three, and you can restrict your search to very specific time periods. This is possible on Ancestry, up to a point, but unless you want a single year or quarter, you can only select +/- 1, 2, 5, 10 or 20 years. You can also search just births, or marriages, or deaths, but not two or all three unless you want to search a lot of Ancestry's other databases at the same time.
After all this you might wonder why I would ever suggest searching these BMDs on Ancestry at all, since FreeBMD seems so much better. And since FreeBMD is free to use, and Ancestry is a commercial site, surely there is no contest? Well, actually there are several reasons for considering Ancestry. First of all, although it is a commercial site, some of its databases are free, including the FreeBMD ones, so cost is not an issue. Ancestry also has a number of features that can be useful for some searches.
When you search for a forename on Ancestry it will return all the results when that name appears in any part of the forename field, while FreeBMD only returns results where it is the first name, not one of the middle names (although FreeBMD will return middle name results if you put + in front of the forename). I have my own Ancestry search pages set to Old Search, with 'Exact matches only' as the default, but if you use New Search you can set a wider range of individual search options for each field. So a search for 'William' on Old Search will return William, William George, George William etc, and the default search option for the forename field on New Search will also return results like George W. There are other settings that you can experiment with, and the surname field offers both phonetic and Soundex options, which bring different results.
Another potentially useful feature of Ancestry is that surname search results will include those where it appears as the mother's maiden name in the birth indexes, although this only begins in the September quarter of 1911. If you are an Ancestry subscriber, results from FreeBMD indexes will be included in searches across multiple databases, which has the value of convenience.
So it's worth considering all the options, even if you are in the habit of using the same one all the time as a matter of habit. I certainly did when I was preparing the blog post. I shall probably use Ancestry a little more than previously, although FreeBMD will remain my first choice most of the time. But no matter which search option you choose, when it comes to ordering certificates make sure that you do this through the GRO's own Certificate Ordering service, or from the local register office, where it will cost £9.25. Anyone else who offers a certificate ordering service has to use the GRO service, so you might as well go direct and save yourself time and money.