Spend the extra money, and get the BH90, so you won't regret it in the future! Make sure any backhoe you purchase is a sub-frame mount only; if you decide to look at other units? You won't have to move your tractor as much for ditching with the BH90, and the extra depth is well worth the cost. You will find many more jobs for the hoe also. The resale value is better if you ever decide to sell it.
What is the primary use of the hoe; landscaping, future home construction, and etc? Is your tractor limited to the BH75 only? If it is future home construction the BH90 will save you a lot of money in a rural environment with septic systems, underground utilities, and many other projects.
Actually, the BH75 has had some problems with bent cylinders. I think Kubota has addressed the issue, but I'm not sure that it was a happy resolution. I'm in the market for a BH for my B7800, and so far the research is showing a third party (Woods) is the way to go. Less money, great quality, great rep, and all the features of the Kubota products. My 2 cents.