Switching group due to a lack of interest; How to break the news to an advisor who has been very supportive
Wow! a really tough situation. On one hand you have the opportunity to continue to be supported by someone whom cares and believes in you. On the other you want to be free to make decisions about your own life without anyone taking it personal. In life people take things personal! Yes, it's not professional but when someone puts time and resources into you they want you to remain committed despite your reservations. 

Yes you could leave your group and go for something you're more passionate about, but would you get the same type of  deal? People talk to each other even if they seem like they don't. Would your new group look at you as a "trader" if they found out you had support and turned your back on your previous team because it got a little rough? Drama, folks love it, thrive off of it and will at your expense. I would say have a heart to heart with your current team let them know of your intentions while remaining politically correct and discuss with the new professor about your reservations to leave your old team but you can be more committed to this new challenge. 

Do what's in you, try and be prepared if things don't work out the way you want. Live with whatever you may regret.