Both the Royal Enfield Guerrilla 450 and the Triumph Speed 400 are excellent first bikes and will serve you well, but they have different riding experiences. The Speed 400 is all about effortless torque and doesn't need a lot of revs to build speed. The Guerrilla 450 also has a punchy midrange, but its engine continues pulling strongly to the redline, and it creates a more engaging riding experience because of that. The Guerrilla also has stiffer suspension, which is great for when you want to attack corners, but over repeated harsh bumps, it can get a little tiring.
Take a test ride of both before you decide.