If suspension is your high priority due to intermittent back pain, both the Yamaha FZS 2.0 and the Yamaha SZ-RR 2.0 can offer relatively comfortable rides, but there are some differences to consider:
Suspension Setup: The Yamaha FZS 2.0 features a front telescopic fork suspension and a rear monocross suspension setup. This suspension combination provides a good balance between comfort and sportiness, absorbing bumps and road irregularities to some extent.
The Yamaha SZ-RR 2.0, on the other hand, also comes with a front telescopic fork suspension but has a simpler rear suspension system. It utilizes a dual-shock setup, which may not provide as much cushioning and shock absorption as the monocross suspension of the FZS 2.0.
Riding Style: The FZS 2.0 is part of Yamaha's FZ series, known for its sporty and aggressive design. It offers a more upright riding posture with slightly forward-leaning ergonomics. This style of riding can put a bit more strain on the back for some riders.
The SZ-RR 2.0, on the other hand, is designed more as a commuter bike with a relaxed riding position, offering a more upright and comfortable posture. This more upright riding stance can help reduce strain on the back and provide better comfort during longer rides.
Considering your back pain issue and the importance of suspension, the Yamaha SZ-RR 2.0 with its more relaxed riding position and emphasis on comfort may be a better choice for you. The slightly simpler suspension setup might not provide the same level of shock absorption as the FZS 2.0, but the more upright riding posture could potentially help alleviate some strain on your back.
It's recommended to test ride both motorcycles, if possible, to get a personal feel for their suspensions and riding ergonomics. Additionally, consulting with a medical professional regarding your intermittent back pain and their recommendations for riding motorcycles would be advisable.
Ultimately, your comfort and well-being should be the top priority when choosing a motorcycle, so take your time, gather information, and make a decision that suits your specific needs and preferences.