Likely you should be able to connect the 3550 to the 3750 stack, but if the devices are next to each other, you might consider replacing the 3550 with another 3750 stack member.
Amit list some advantages of a chassis, but those he lists are not quite fully unique to a chassis relative to a 3750 (or 3750-E) stack, except perhaps for #3 on the issue of bandwidth scalability. There are some others, such as the 6500 series supporting service modules or WAN modules, additional software features, IOS stability, but the chassis is often more expensive, so stack vs. chassis can be a difficult choice unless you can carefully compare their differences and what's important to you.
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...