What an odd question lol... I know we are all gamers here, but in this particular context, YOU are the player solomio, it's up to you as an individual how you decide to enjoy a game, if you feel you are done with DMC1 and no longer want to play it, then feel free to move onto DMC2 right away, and if you loved the game and want to play it again with a slightly steeper challenge, then hell, do that. There is no "gamers code" forcing you to do anything, it's all down to you. DMC does have alot of depth and fantastic replay value, but you shouldn't feel forced into playing the harder modes just cause thats what everyone else does, and if you wanted to play the harder modes in order to prepare you for DMC2, I wouldn't worry about that at all, DMC2 is a far easier game than 1, if you can do easy auto DMC1, DMC2 won't be beyond your skill level.
However, at the risk of undermining everything I said about personal choice lol, I will add that completing DMC on its easiest setting is not the way to get the most out of the game, and not really the way it is intended to be enjoyed. Leaving DMC1 now, you probably haven't experienced all it has to offer. I would also be shocked if you have all the moves etc unlocked in the space of one easy auto playthrough. I personally played the DMC games when they were released, so there was no "I'll move on to this one now" available lol, but I didn't stop playing each of them until I had unlocked everything, all the moves, modes, costumes etc etc and completed it on all difficulties (I nearly called it quits during DMD, but eventually picked it up a week later and finished that off) I personally left it there(aside from the occaisonal play session of course) as it felt like I was finished with the game, others might decide to go on playing until they had done all 9999 levels of BP and SSed every mode. (basically shooting for 100% completion rather than settling for "finishing" the game)
But Overall: Stop making it complex! It's simple! Play the game until you don't want to anymore! ;) How long I will play a game is obviously determined by how much I like playing it, not a code of conduct or beleif system. Definitely don't force yourself through things you don't wanna do just to earn the title "gaming enthusiast" lol But at the same time, don't burn your way to DMC3 just to get to the finish line, feel free to take you time to enjoy 1 and er.. 2, properly as well.