Hi Sslhp
I've got both the 150's and the 180's, actually I've got the baregrip 200's too. All my runs less than 10 miles are done exclusively in 1 of these pairs of shoes and I've done way over 150 miles in each shoe.
Seen as though your question is focused towrds the 150 and 180 I'll direct my response to those.
First up; the Inov8 website says that they are built on the same last but I'm not convinced of this as the 180 is much longer and marginally narrower than the 150.
Even though the 180 is heavier it is by far the most minimal of the two as it uses a durable rubber compound in the whole outsole which, up untill now, is holding up ver nicely. Because its only the outsole between your foot and the ground these shoes are pretty unforgiving if coming from a minimalist shoe like a Kinvara or Nike Free 3.0; having said that I think they are absolutly brilliant.
The 150 uses a fairly thick midsole material with an integrated outsole; as the sole is less dense than the 180's you actually get lots more cushioning for a lighter weight shoe. unfortunately, if your form is a little bit off the soles tend to wear excessively. If your coming from the Kinvara/Free 3.0 type shoe i reackon the 150 is a perfect introduction to a zero drop road shoe....but work on your form.
Other things; the lace system on the 150's is brilliant, the laces on the 180's are about a foot too long. A UK11 150 weights 214g, a UK11 180 weights 230g.
If you want any other specific comparisons give me a shout.