Here is my honest opinion.
A tethered solution is never going to give you the exact performance a straight up aircard will.
If maximum performance is a non-negotiable...Aircards rule here.
Another thing I don't like about tether- Can't be on the phone and online at the same time.
But...value of speed for the price- tether rules.
Flip a coin?
Let me put it this way.
Here's a senerio-
I have 10mb home internet (cable),
Sierra 595 aircard,
and a tethered pearl.
I would only connect my Pearl if the power was out, and I lost my aircard.