Hi Maria,
Great question. I've been comparing the two products as well and here's where I see the biggest differences:
1. Delivery of the product: Fluenz is physical discs that you plug in and use like Rosetta Stone. Yoyo Chinese: all videos and materials are online organized into different levels
2. Access: Fluenz has 3 different levels that you have to buy if you want to reach the maximum level with them. Yoyo Chinese: you pay a monthly subscription price based on 1, 3, 6, or 12 months and you get full access to every course on the site. You can jump around to different videos or search for specific phrases you want to learn even if they are not up next on the course schedule.
3. Course schedule: Since Fluenz is CD-ROM software, you have to start at the beginning of the disk everytime. Yoyo Chinese: you can bookmark your progress in the Course Schedule and pick up right where you left off.
4. Updates: Fluenz has created the discs so these are hard copies, there is no updating or changing of any possible errors, or making videos more timely. They just have level 1, 2, and 3. Yoyo Chinese: constant updates as courses are being further developed. There is no end to how many courses will be offered. It's just a matter of time.
5. Here's a big one: customer and student support. Fluenz does not have an interactive team or forum where you can ask questions as they arise during your language study journey. Yoyo Chinese has Chinese teachers on staff who answer any question you may have within 24 hours. Also, they are very active on social media with supplementary words, vocabulary, interesting updates,e tc. So you are never stuck out their alone in your language learning adventure.
6. Price: Fluenz is $298 for level 1+2+3. There isn't a trial period. You are just buying CD-ROM discs and you better hope that you finish all the discs, otherwise you've wasted a lot of money. Yoyo Chinese's subscription model is more attractive, because you can try it out online for a shorter period of time. If you love it, you can extend your membership. They also run specials pretty often too. And members always get 10% off when renewing.
7. Company philosophy: Both company philosophies are similar: there are teachers who are passionate about their own language and want to share it with the world. The only difference is that Fluenz is in the Language business, and Yoyo Chinese is in the Chinese language business. So you can be sure that Yoyo Chinese will have their full attention focused on making your Chinese learning experience the best it can be.
Oh, and I can also agree with
@Ruthikins, the Chinese Grammar Wiki is really useful too! But it may not be useful unless you've been studying the basics for a while.
I hope this run-down helps your decision!