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Stainless Steel, is a great mod - with many factions that differ to DLV, however the amount of mini-mods that there are and complications with installing will leave you finding you need to re-install god knows how many times. The battles in Stainless Steel have more depth because there are more variety of units, and more custom battle map locations to choose from. It also has a large modding community with itself with it's followers trying to make the game better.
Campaign-wise, Stainless Steel also focuses more on the the middle east and west Asia. The Americas have been removed from the game so no crossing the Atlantic to find angry aztecs

. However because of this, there are new factions such as the Order of the Knights Templar (fantasy), and the Cumans etc. The maps stretches across to far west Persia.
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Deus Lo Vult, the ultimate Role-playing mod for M2:TW, the campaign has far more depth than any other mod that exists, as well as User friendly tutorials when first starting up for new DLV players. It is
extremley hard and will leave you feeling battered and bruised but you will be happy

. There are so many Role-playing elements it's impossible to name them all, but from finding legendary swords, to crowning - promoting your generals through the ranks to locking them in dungeon - DLV will test your metal. Keeping the Americas (but actually accurate this time) and of course and without restrictions you can cross them early in the game as any of the factions, and develop an economic Powerhouse - providing you can actually destroy the Aztec tribes. If you do decide to download DLV, it's best to choose the Historical campaign
I'd download the Open Beta's if I were you since they are by far the best to date.
The choice is down to you at the end of the day,