I had my HP LTO-3 external half-height drive repaired by Midwest Technical, after HP told me it was a lost cause. I asked their tech which brands/types of drives to buy/avoid. He commented that full-height drives are built much better.
Half-height drives are much less expensive. The HP list price for the full height 6650 is $5700; for the half-height 6250, $3150.
I'd be curious as to what other people think. The tech at Midwest Technical may rarely not see many full-height drives simply because they are rarely sold.
In the days of LTO-4 and earlier, the Full height drives provided an average of 50% better throughput than the half height versions of the same drive. This was because the mechanicals were larger and more robust and there could be more sensors installed in the tape path to monitor and control tape speed and positioning. Since LTO-5's second update, the mechanical differentiation has become NIL and the actual tape path components are far more "exact" than in prior versions.
You are correct about the ratio of full height to half height. Even HPs' internal numbers (were they public) would probably match ours of over 22:1.
In the current (2014) LTO-5 and LTO-6 world, we (TOLIS Group) don't even sell the full height devices except for use in 3rd party libraries that were only designed for full height drives. They bring absolutely nothing to the table. In fact, we even designed and had built our own enclosure that uses the HH device and provides space for either 2 tape drives or a drive and a 2.5"x4 drive disk-cage for more reliable cooling and MUCH quieter desktop operations.
Save your money and buy half height. And, remember that an LTO-5 drive will read / restore from LTO-3 tapes. so let that LTO-3 RIP.