For me, two issues sent me in the direction of Speedo Blackline.
1. Reliability
In all my years assisting, I'd never seen a Speedo pack fail on a job or cause anyone any harm (like blow up, catch fire, etc.) If you have reasonable backups (tubes, model lamps), you can make it through the job.
2. Availability of rental accessories
Odd reflectors, specialty heads, etc...the rental houses in NY, Chicago, LA have always been ready to give me anything in the Speedo line.
It's heavy stuff, but very robust, and when something does fail it's been relatively cheap to fix. All of the above also holds true (in my experience) for Norman, and to a lesser extent Dynalite. Norman is more popular on the westcoast than the eastcoast, but still the rental support is there almost everywhere I've tried.
Also, the resale value in Speedo is consistent. If you buy gear and it's not for you, in my experience you can get out of it what you put into it.
YMMV,
Neal