My MiniMax slider is 8'6" and that extra few inches means a full 8' rip can be done easily. You'll need to confirm with the manufacturer of the machines you're considering what their actual capacity is because sometimes that's not evident or accurate from just a simple description. They should be able to tell you what you can get with full wagon travel combined with the blade size for the machine(s) in question.
That said, how often does a project require a full 8' rip outside of sheet goods? I can only think of one project that I've ever done that required a piece of solid stock longer than that--the mantle I made for our large, walk-in fireplace in the great room--and I just did a conventional rip with the fence to accomodate that need. The majority of straight line ripping I do is pieces that are less than 6' and often just 3-5' long.
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The most expensive tool is the one you buy "cheaply" and often...