If you've got nothing wrong with the J7s, keep them.
I've run both, I have WAY more time on the J7s. I'm very happy with the performance. They're easy to service too. Maguras are a little more finicky to bleed. It's also less likely that shops will have Magura parts in stock. The shops I work at always stock Avid and Hayes parts. I have no complaints with Magura otherwise.
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