To some degree it is just semantics as travel gas = first deco gas for most deep OW trimix diving situations where you start your deco fairly deep. The major difference would be at what depth a diver decides to make the switch and on a fast descent to 100', it makes very little difference in terms of how much of what gas is used.Its less than 15%. You might pass out after a LONG time breathing 15%, otherwise you just get winded and suck down your expensive trimix on the surface for no reason. Climbers can acclimatize to <15% without too much trouble at all.
Travel gas is really old skool thinking, many people just descend fast. Or breath a deco gas for the upper 20ft. Bringing a whole seperate cylinder just to descend a little isn't as popular nowadays as it was in days of yesteryear.
One argument for using back gas from the surface is that it eliminates a gas switch and eliminates the potential to switch to the wrong gas - and also eliminates the work of stowing a stage reg.
On the other hand, a hard swim in strong current from the stern to the anchor line on the bow has caused more than one diver to go unconsious on mixes lower than 16%. Consequently, you need to weigh the costs and benefits of one approach against the other depending on the circumstances.
The "really old school thinking" comment however is not accurate.
Travel gas can still be "travel gas" in a cave. Consider a dive in P3 to Hendly's Castle. You have 30-35 minutes of swimming at depths of 50-60' to get to the jump where you drop down to 135 feet then after a few more minutes passing through the sand slide, you drop further to 185'.
Doing 60-70 minutes in and out on bottom mix with a 190' MOD just to get to/from the jump where you can go deep makes no sense. Similarly, doing the eventual deco on a 32-36% mix you'd normally use at 50'-60' also makes little sense.
At the other PO2 extreme, using 50% for the travel gas pushes the CNS % a bit too high, uses a PPO2 of 1.4 for long periods while swimming with stages, and is then also less than ideal for the majority of the deco.
So you end up doing the dive with an O2 bottle for deco, an 80 with a travel mix in the 30-36% range and bottom mix in the back gas.
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