The 150qt cooler would be great when you're making over 10 gallon batches... You could easily mash about 90# of grain in that thing (that's a LOT of grain)...
For most 5 gallon batches a 40-52qt cooler would be a good size. Once you start looking to make either really big brews, or move to 10 gallon batches, then the ~70qt cooler mash tun makes a lot of sense. I think that the grain bill for a 5 gallon batch, even a barley wine, would get lost in a 150qt cooler.
I started with a 70qt cooler, but have since also picked up a 10 gallon (round, RubberMaid) cooler to use too. Now I can minimize the dead space in the cooler, above the grain bed, for any batch I'll make over the next several years. I have the flexibility to make any OG 5 gallon batch, as well as 10 gallon batch (the 70qt can hold over 40# of grain), that I'll want to. Hell, I can even mash for a moderate OG 20 gallon batch with the 70qt cooler (about 5.6% ABV batch)...
If you already have the 150qt cooler, I would hold onto it for later. I would look at how you're brewing now, and when you think you'll get into 10 gallon batches (or larger) or want/need to mash 25#+ of grain... If that's a long way off, then go with a 40-52qt cooler mash tun... Keep in mind, you can make a BIG barley wine (5 gallon batch) with that much grain...