My rules are my own adaptation of classic Piquet, wholey re-written to my own (and group's) tastes, specifically for large battles with 24+ units (typically) a side. We've played successful games with 40+ units a side using them.
My views on howitzers in most rules is that they are either over complex or ignored / nodded to. I've done my best to make them different whilst keeping game play reasonably simple. The differences are illustrated below:
Example, a field gun (medium positional artillery) with a D8 standard fire factor has four ranges with modifiers, these are:
Close (canister) 12" Up 2, effective 12" – 24" No change, long 24" – 36" Down 1, and extreme 36" – 60" Down 2.
Field guns are pretty standard direct fire weapons so the usual kinds of adjustments are also made for cover, target formation, etc.
I view howitzers as having less punch and range, and at effective range the common munition would be shell with it's added complications. Consequently I found range bands the easiest way of adjusting fire within the basic mechanisms (Long range is always down 1, Extreme range is always down 2, to make remembering things easy). Howitzers are generally counted as medium artillery so the starting fire factor is still D8.
Close (canister) 12" Up 2, Long 12" – 36" Down 1, Extreme 36" – 48" Down 2.
Note they do not have a 'no change effective range' band and are immediately at long range over canister range; this represents the general difficulties with firing fused shell at comparatively high angles.
Howitzers have some advantages though. They don't count cover; although slightly blanket, this makes howitzers good Vs troops behind linear obstacles (high angle fire) and in buildings (exploding rounds in small spaces) and have 1 in 6 chance of causing fire in buildings. Howitzers also get an up one Vs horse when not using canister (scary projectiles). We also allow any clear LOS to enable shell to be used over the heads of friends – e.g. from any higher elevation or to tall targets such as buildings, wood edges, etc. You still can't fire over the heads of friends if the gunners can't see the target because of them – they don't have FOOs.
All in all, this makes howitzers inferior to standard artillery with open fields of fire, and standard artillery has greater range overall, but howitzers are better against certain targets under certain conditions.
Players here tend to choose standard artillery when they can unless they have a particular need of shell firing howitzers – probably how it should be.