EuroJackpot and EuroMillions are both multi-country European lotteries, which leads a lot of players to assume they're the same game under two names. They're not — they're run by different groups of national lottery operators, with their own number formats, participating countries and jackpot rules.
Different operators, different countries
EuroMillions is run by a group of Western European operators including the UK, France and Spain among others. EuroJackpot is a separate pan-European game with its own, partly overlapping, group of participating countries, largely centered on Central and Northern Europe. A country can participate in one, both, or neither, depending on which consortium its national lottery has joined.
Number formats aren't interchangeable
Each game has its own pick-numbers-plus-extra-balls format and its own ranges, so a "winning combination" for one game means nothing for the other — always play through the specific operator or licensed agent for the game you actually want, and double check which one a ticket is for. Use this site's number generator with the correct lottery selected to avoid mixing the two up.
Jackpot caps
Both games set a maximum jackpot cap. Once a jackpot reaches that cap, additional rollovers stop increasing the top prize and instead flow down to lower prize tiers, or trigger a "must be won" draw depending on the specific game's rules at the time. Because the two games set their own caps independently, one can be sitting near its maximum jackpot while the other has plenty of room left to grow.
Which is "better"?
Neither is objectively better — they're simply different draws with different odds, formats and availability depending on where you live. Check the current results and next draw for both on this site before deciding which to follow.