You do not need a cooltube with the 150's, just fans blowing on the bulbs, the same fan you have to blow the tops of the plants and then if your current exhaust is enough now it'll be enough for 300w of hid as well. If he's running 312w of cfl now then 300w (two 150's) is the way to go for sure. One 150 might not be enough of an improvement though he would save half the energy if that mattered and work out about equal.
If you want to use the whole height of 60" you mentioned later and grow tall sativas or something then you could cram in a 400 with a cooltube maybe, but otherwise ***two 150's is ideal***.
Econolight sells the 150's for $20 ea complete with bulbs and ballasts, with shipping you'd be looking at around $55 for two. Try and try hard to beat that.
I'll give you the link if you can't find it, it's all over this site, we've posted it many many times now, many people are using them.
Landragon is wrong about vertical 250 in a cooltube being the way to go... No, you'd have to convince me. A 150 on each side horizontally in a normal arrangement is sure to do better.
For the best results I suggest you leave the cab as is and veg in the extra space with a couple flouro's rather than devote all the space to flowering, but if you will veg in another cab then that's up to you. Only a true sativa will use 60" height in the confines of your small cab, so maybe that will help you decide.