Once the Hull Upgrade is available I suspect it will be used for a number of things. There has just been a big discussion on whether or not the Shield Upgrade is better/worse than the Stealth Device but with the Hull Upgrade coming in a point cheaper than the SU that discussion could turn to favor the HU. HU vs SU is mostly just looking at the ability to possibly negate one crit for 33% more cost. If you say the SU is better than the SD then the HU is certainly better than the SD.
When a ship naturally has shields I can already see why the Hull Upgrade will get used. Although it depends on the dial you could look at a Z-95 Bandit with HU as an X-Wing Rookie with one less attack die but costing 15 points instead of 21. That's almost the same [ercentage jump you see between a TIE Fighter and Interceptor except the Interceptor has boost and a better dial.
Given the choice between Hull Upgrade and stealth device on an already agile ship, I will ALWAYS take stealth device still. For the same price, HU lengthens the life of your ship by one damage. Stealth Device is still superior in that its effect can significantly lengthen the life of your ship, possibly through several rounds.
Hull upgrade will see its uses as a cheaper alternative to Shield Upgrade on ships with low agility where stealth device won't necessarily help because they are likely to take the damage anyway. Ships with 1 agility stock don't get much out of Stealth Device because most incoming attacks involve 3 or more dice, most or all of which could hit. Therefore, for the same cost, you have negated one hit more than you would have for this singular attack, but Stealth Device proved no better than Hull upgrade for the cost. There ARE specific uses for hull upgrade that improve its use, such as with Chewbacca in the pilot seat. As far as Chewie is concerned, a Hull Upgrade is just a cheaper way to get another hit point.
Therefore, each card here has its uses in list building. Shield upgrade is an expensive single hit negation for any ship, but particularly low agility ships. Hull upgrade is a slightly cheaper card with a similar function, but it adds the possibility of a Critical Hit with all pilots but Chewbacca. Stealth Device is a better option than either for ships with high stock agility, IE TIE Fighters, interceptors, X-Wings with R2-F2, etc. Imagine an E-wing, with 3 stock agility and an astromech slot. R2-F2 and a Stealth Device give it five defense dice, and with PtL or a focus pass from Garven or Dutch, it will be nigh unhittable.
So those are my opinions on each of these modifications. To put it more simply, this is typically the order that I would consider them in.
Stealth Device>Shield Upgrade>Hull Upgrade