This, like many other questions similar to it, can only be answered by first defining your requirements.
There is no single answer to "Is Juniper better than SonicWALL" or "is Juniper better than Cisco" or "Is Cisco better than CheckPoint"... because every one of those vendors is going to have some strengths and deficiencies.
Only by defining your particular criteria can an informed decision truly be reached. I am *not* a fan of unwavering vendor loyalty. The best tool for the job is the best tool for the job, I don't care whose name is printed on the front of it.
Here are a few things to think about...
Are you looking to do "next-generation" firewall services, such as application ID/firewalling? SonicWALL has been doing this for a long time and has a very well-established technology. Juniper is just breaking into the AppID/AppFW segment, and it's kind of a kludge (if you ask me) on the SRX, *especially* the branch SRX.
Are you looking for robust enterprise features? Advanced routing and/or tunneling options? Both solutions will handle your typical IPsec scenarios, but with the SRX you have the flexibility of policy-based or route-based VPNs. SRX is going to handle things like BGP, GRE, virtual routers, even MPLS, where the SonicWALL is not.
How many firewalls are you looking at deploying? Do you need centralized management and reporting, etc. for multiple boxes from a single place? If so, SonicWALL's GMS product beats the pants off of NSM, no question about it.
As far as H/A goes -- the SRX and the SonicWALLs are going to approach that differently. Sonic is going to give you a good break on pricing for H/A pairs -- you typically only pay half for the H/A box. SRX has had some serious growing pains with clusters. They're better now, but it was never a "graceful" implementation by any means.
If you like GUIs, the SonicWALL has a darn good one. The web UI is very fast and responsive. J-Web, even with all the recent changes and improvements, is still extremely slow and clunky by comparison. SonicWALL really did a much better job of prioritizing management functions in Core 0 and it shows. However, if you don't care about GUIs and you want a CLI, then SRX is your choice -- the SonicWALLs don't even have a CLI (at least not one where you can configure the box as far as address objects, policies, security settings, etc.) The newest SonicOS is introducing a "real" CLI -- but this is in its infancy and is only barely available (if at all in general release. I've seen it on their "SuperMassive" boxes...) That being said, the SonicWALL was designed from day 1, from the ground-up, around their GUI. So, with J-Web, you get a box that's built around a CLI and the Web GUI is bolted on top, and there are constantly things where "you can't do that through J-Web, you have to go down to the CLI to do that"... with the SonicWALL, every function of the box's configuration is done through the GUI. Period. It's just a matter of preference there as to how you prefer to work.
Both the branch SRX and the SonicWALL systems are based on a similar hardware architecture, though not exactly the same. Both products use the Cavium multi-core CPUs for security services and traffic processing. I will say this -- SonicWALL has many more years of experience programming for those CPUs and their ability to write software that is tight and optimized on those platforms is obvious -- they achieve better throughput on the same class of chips compared to the SRX, and their management functions are WAY faster (they use Core 0 for this much more efficiently than the SRX).
Generally speaking, for similar performance, the SonicWALL is usually going to cost a little less, but the SRX is going to give you more "enterprise-class" features. Decide which of the features and functions are important for your particular implementation and focus on the needs that are applicable to you. Don't get hung up on features being in a product if you're not going to use them. That's a Cisco thing -- they'll sell you a product that will cook your breakfast, fix a flat tire on your car, navigate an airplane, and plant trees in third-world countries -- and oh, by the way, they route packets too. You end up paying for a whole LOT of stuff that most people never use.
Make yourself a requirements document. Categorize things into "must have" and "would be nice to have." Any criteria you can think of, write it down on that document. Possibly assign a weight to items. Most importantly, invite the vendors to come talk to you. Utilize the channels and resources that are there. Juniper and SonicWALL would both be happy to have a sales rep visit you, and they should bring an SE (Sales Engineer) with them. Use your requirements document and have the team answer your questions and tell you about their product, and why they feel it's a better solution for you. Always remember Keith's Law of IT: Vendors Lie. Don't get caught up in marketing hype -- take the information they give you and evaluate it and do some of your own research to validate it. Read third-party reviews and independent tests ("independent" is a bit of a stretch...). Check things like Gartner and NSS reports.