I have used SEL, and they're fine, probably one of the best. I think GE's Multilin family of UR "Universal Relays", which are designed on a modular platform, are interesting...especially with the hard-fiber process bus "brick". The brick is a remote I/O device that would mount in substation equipment, say in a circuit breaker cabinet, that uses IEC 61850 GOOSE messaging. IEC 61850 is probably what everyone will be using sooner than later for a protocol. It's an advanced protocol developed specifically for utility applications and reduces time spent mapping registers or "objects" by doing it almost automatically.
The GE brick is a remote I/O interface where even CT and PT inputs can be plugged into it, and the UR's simply sample the signals at their own internal clocks. No other protective relay manufacturer has a remote I/O device like this, of which I know, on the market right now. The brick can significantly reduce copper wiring in the substation yard, replacing copper control and instrument cables with fiber optic cables that are immune to EMI. One can even use redundant bricks, have primary and backup relays on their own brick and have those two bricks "talk" to each other; then one can shift the backup on the primary brick if that secondary brick fails or if it's being serviced. ABB has something similar to this called a "merging unit", but I don't know if they have it widely available, yet. GE Multilin makes relays for nearly all applications, not just power plants. It's just that many companies use them for plant applications. However, I see no reason why they couldn't be used on an entire transmission system, again, as Multilin has relays for about every application including buses that could use dynamic zone protection. They even have transmission line relays (directional distance, mho or quadilateral characteristics or pilot relaying).
I would seriously look into the GE brick. It would all but eliminate copper control and instrumentation cables in the substation yard. Really you'd only have to run DC power to the brick and that's it....of course you'd have to run power to the breakers, transformers, etc. Programming probably wouldn't be that bad, too...probably about the same as using conventional I/O on the back of a numerical SEL relay, except it would all be via GOOSE messaging. I think the brick takes advantage of IEC 61850 very well. Some people may be hesitant to use this because important I/O, including tripping outputs, for the relays wouldn't be hardwired to the relay; relying instead upon a communications channel. However, I've not heard bad things about the reliability of IEC 61850 or the GE brick. Also, the brick is environmentally hardened and the fiber cables running to the brick are rugged and even have a rodent barrier built into them. Also, it seems more utilities are turning toward IEC 61850 for protective relaying applications. So, if you're uncomfortable with the idea of using a comms channel for crucial I/O, I'd probably go with SEL or GE. Either are better than most other manufacturers out there for either plant or transmission relaying, with good support and customer service....SEL's support and customer service might be better than Multilin's.