Those will be cheaper and/or more powerful. If that's all you care about, then the Steam Machine probably isn't worth the price or wait.
But they'll also be bigger, louder, probably not support HDMI-CEC, may or may not support waking via bluetooth, and it's a gamble whether they'll support Linux as well. Both are totally valid options, it just comes down to which tradeoffs you prefer.
The "boring" business boxes from the usual suspects are usually well supported by Linux, even if HDMI-CEC and BBT wake might not be priorities for the manufacturers. The hardware is so unimaginative it's almost an Intel or AMD reference design.
But they'll also be bigger, louder, probably not support HDMI-CEC, may or may not support waking via bluetooth, and it's a gamble whether they'll support Linux as well. Both are totally valid options, it just comes down to which tradeoffs you prefer.