MyHeritage is better for continental Europe. In my limited expedience Ancestry has close to no coverage for Germany and Scandinavia. Almost all matches for such lineages will likely be from immigrants, mainly to the US.
The main benefit of Ancestry is the database size -
https://isogg.org/wiki/Autosomal_DNA_testing_comparison_chartAncestry at 27m and MyHeritage at 9m.
With Ancestry's skew to tests in the US, Canada, UK, Ireland, Australia and New Zealand, it means people with heritage US and British ancestry will get far many more matches on Ancestry. You will find even higher number of matches if you have Jewish ancestry and Mormon ancestry or relatives. As those people take so many tests.
Then there is the situation with Ancestry's timber system, which strips out what they believe are false positives. The rate of false positives on MyHeritage is around 70-80%. So the majority of your matches are not reasonably close relatives, but people who share a common region of ancestry. This seriously hampers finding real DNA relatives, as if you find common ancestry in a cluster of matches on MyHeritage, you have a high chance they are not actually closer DNA relatives. I'd say the false positives on Ancestry >= 30cM are 0%. Below that it's difficult to tell, but it's much less than MyHeritage.
So if you have US, British ancestry you will be able to find multiples more matches, in the range of 40X-50X, with a high degree of confidence. MyHeritage is useful for closer matches, mostly above 100cm. Below after around that level you get false positives.
The MyHeritage cM values are also less helpful, because they don't strip out identical by state (false positive) segments. Below 100cM matches, you need to multiply the cM by 0.6 to get a closer idea of the identical by descent shared cM. So for a typical US or British tester, by the time you've got through your 1st page of DNA matches, you are probably down to people who are 4th-5th cousins. By the time you are through to page 7-10, you are dealing with people with whom you share about 15cM, which might be identical by descent, but is more likely to not be. And 15cM is a very ambiguous amount, as it's at the threshold where it's not not uncommon for 15cM to be inherited (nearly) unbroken over multiple generations. Around 15cM is the average size of segments inherited from either parent.
In terms of Ancestry:
>= 30cm - you will likely be able to find a relationship if there is no NPE, a tree and no brick walls, relationships usually quite predictable
20-29cM - these sizes can be much more ambiguous in terms of relationship, owing to the possibility of smaller segments being inherited (mostly) unbroken
13-19cM - progressively more ambiguous
8-12cM - the range where segments could potentially push outside of the reasonably feasible to research window of ancestry
While you probably only have 10-20 >= 30cM identical by descent on MyHeritage, on Ancestry you'll have more like 200.With Ancestry, you will typically get several to about 50 matches in a cluster, even for 4th-5th cousins. With multiple people related in the main window that is not so difficult to research (1775-). You look through their trees to find common surnames, places and ultimate ancestors. Heritage just doesn't have enough matches to be able to do that very consistently. Since many matches will have no tree, NPE, or dead ends that aren't easy/possible to solve.
If I go through my Ancestry matches down to about 20cM, if you exclude the ones that have no tree or not enough of on to reasonably find out their ancestry, I know how about 75% are related to me, or I have them in files where common ancestors have been found among the matches. (I have three NPE lines and a suspected adoption, which most of those relate to). Yet if I go through the MyHeritage matches down to Ancestry-comparable 20cM, I don't know how I am related to 90% of them. For the vast majority of them I don't even known what side (paternal, maternal) I am related to them on, never mind the actual line of ancestry. While with Ancestry, for 20cM+ matches, I know which line of ancestry they relate to in most cases. Owing to Ancestry having many more matches.
With Ancestry DNA I've been able to break down 7-8 brick walls, prove most of my lines back to between about 1650-1775, identified a likely adoption, have solid leads and some known ancestors for three NPEs, and expect to be able to solve more lines into the future. Also solved the paternity of a friend's paternal grandfather (who it turns out was my grandfather's half 3rd cousin) and united a few unknown half-siblings. Comparing to MyHeritage, I've just got matches who have provided no clues to break down brick walls, and an unknown ~2nd cousin match where I don't even known if they are a paternal or maternal match.
That's why Ancestry wins hands down, and the competition are not close. Despite their bleeding of their loyal customers and having developed their system very little.
The best process for getting the most out of genealogy DNA is:
1) test with Ancestry
2) upload that test to MyHeritage + FTDNA for free
3) test with 23AndMe (not sure what will happen with that)
4) systematically go through the matches to find common ancestors among them