I used Bullseye (or greenhouse occasionally)
Started fusing about 11 years ago, so there was little choice in the UK for supplies, this was what my tutor used, so I did. Was always impressed by the supplier (Warm Glass mainly) and by Bullseye's technical support and information sharing with their tech notes and stuff. I once got a phone call from Portland, Oregon. It was one of the Bullseye staff asking me if I found the staff support on the forum helpful and did they give the right sort of information, etc. They seem to do a lot of ongoing research and development all the time.
To be honest, I have nothing to compare it with. Many say Spectrum is/was easier to cut, which I presume is because it's machine rolled rather than hand-rolled like Bullseye mostly is which was why it was always cheaper, and sometimes I see a glass that is so lovely and is NOT Bullseye! However, I seem to also remember seeing comments that Spectrum (or S96) wasn't as stable as B/Eye, I don't know if that's true, or if they just hadn't tested multiple firings and published their results.
And as I am not a S96 user, I have never really sought out that information, so it might have been out there too.
I don't know if the Spectrum range will remain cheaper, as the company shut down as it was not financially viable - I think they tried to turn art glass into mass production with more machine rolling, and then with all the issues we've had over the last few years with downturn in trading globally, banking crises, etc, followed by the emissions restrictions, the business wasn't sustainable. I understood someone has taken over all the production machinery, I didn't know it was moving to Mexico, and of course there are other products of Coe96 glass anyway (not that COE is a guarantee of compatibility anyway).
So, I started with Bullseye for no other reason than it was what I was given in my starter pack! Because it is the more expensive of the range, I did consider Spectrum at one point, but realised that although I would make savings on sheet glass, I'd need to buy that many accessories such as frit and stringer that I already had such a choice of, then the issue of efficient storage and space to keep them apart (and with running classes, making sure students don't mix them up) that it probably wasn't worth it.
Yes, it's more expensive, but it never has compatibility issues, they do extensive testing, research, development, there is masses of widely available information out there and I think their willingness to publish what they do is worth that extra cost. They say you get what you pay for. Oh, and of course, I also know it well enough through experience to know ideal firing temperatures to get the effects and results I want without having to look up schedules.
As Flame'n'Fuse says, you might be driven by location - I am miles and miles away from any fused glass supplier, again if their had been a local shop, it might have influenced me, but as it is, I have to buy all mine online or do a five or six hour round trip to get it from the shop myself. So, again for me, good websites and efficient, friendly, helpful suppliers with a good choice available and who package well and send out orders quickly (but not too expensively!) are a must.
If you've not started with any, and depending on your location, you could also consider float compatible glass, although not sure what the availability is of accessory and coloured glass.