Personally, I've been playing a mine throwing/AF regulus more lately. At level 2, I put a point into the first range upgrade and never touch it again. At level 9, I stick a point into snipe. Eventually I take snipe up to 3rd or 4th tier, but Generally I max out attributes instead.
But lately I've been wondering if it wouldn't be better to simply forgo Snipe altogether for range upgrades.
Snipe as an ability has two uses, imho. First, and most often, as a finishing move. Either you are cherry picking someone else's DG (either before they can do it, or else if they can't do it/in retreat), or you are kissing your own Target a fond farewell. This is very effective when enemy DG's use a scroll to port out - as some have noted, as long as you start the Snipe with the target in range, the shot will track and hit the target, no matter where they port to. Think of it as a blue turtle shell in Mario Kart.
Second, Snipe can be used as an opening, generally for another DG - the opponent may feel confident that they can withstand an opening salvo, but not consider the possibility of a Snipe from outside of visual range landing right before a combo. At the least, it may persuade a DG to retreat, rather than chase one of your wounded allies.
The first upgrade is pretty obvious in it's benefit - for one, you can stand further away, but this is most noticeable when attacking Orb and Archer towers (allowing you to hit from out of counter attack range).
But range has another benefit as well; It extends the time that Regulus can DoT a given DG. By having a larger range, you can fire on enemy DG's sooner, and continue to do so longer as they flee. Although I'm not totally sure of how the numbers presented in the game work, if we make some basic assumptions, I think the point can be illustrated.
You have a move of 6.0 (assumed to mean 6 units per sec), range of 15 (assumped to reference the same units as movement), with damage of 150 (keep in mind, these are numbers used purely for hypothetical purposes), and Attack speed of 1.0 (assumed to mean 1 sec in between each attack)
Enemy has a move of 6.0, range of Melee (3?)
From the moment the enemy enters your range, you begin firing. It takes the enemy 2 seconds to enter melee range and engage you - you have fired twice for 300 damage. As he melees, you are firing. All things being equal (HP, Armor, MP), he will begin his retreat before you need to. You will get another 2 shots for 300 dmg in without being countered.
At this point, giving chase is really tempting to try to finish the kill... but folly. Once the enemy is out of range, as long as they move in a straight line, they will always be out of range - because the two of you are moving the same distance in the same amount of time. It's only if the enemy DG makes a turn, or for some reason stops momentarily, will you be able to get off another shot.
However, if your Range were 17, you could almost get off an extra 2 shots (another 300 dmg) - one coming in, one going out. It's also that much easier to catch the DG if you give chase. If your range were 19, those extra 2 are guaranteed.
So in many ways, Snipe and Range boost seem about even...
+Snipe vs. Range - Early game+
Early game is where Snipe tends to outshine range boosts. The reason is simple - enemy DG (in my experience) tend to Walk-In, Port-Out (as opposed to Walk-Out). The 'Hit Anywhere' finishing ability of Snipe is much more useful (as well as the lump dmg for the lower HP values) than Range... because you only get the Walk-In dmg bonus. In reality, all this does is simply make the DG Port-Out sooner.
+Snipe vs. Range - Mid Game+
Range starts to show it's value (I think... need to do some testing, so consider this theorycrafting). The reason is again simple - the value of range depends entirely on another factor: DPS. The more you can reduce your attack speed and the higher you cam pump your per shot dmg, the greater the advantage afforded by being able to fire sooner and longer is. By midgame, you'll have more gold (granting access to these items, as well as some -movement speed on target) as well as more points in AF.
+Snipe vs. Range - Late Game+
Here is where I think Range completely outshines Snipe. Unless you have concentrated snipes from multiple Regulii, it just doesn't do enough dmg. Additionally, game dynamics change - everyone has access to better items, and movement tends to be Port-In, Walk-Out (as Regulus, you can also force this behavior - remember, there is a long cooldown on scrolls, but you can burst grenades, mark, grenades, af shots to bring a DG into retreat-HP before the recast is up)
When enemies Walk-Out, Snipe doesn't travel as far, and doesn't gain as much bonus dmg.
However, at this time, it's possible to get Regulus' dmg in the 480-540 range, wither ~0.50 attack speed. Even an extra second of shooting is about 1000 damage extra (not to mention on-hit effects). In most cases, that's sufficient to kill a fleeing DG (even if they are trying to Port-Out). Thats at least comparable to the damage Snipe can do, but that bonus is always "on". Moreover, it's easier to attack towers and even forts from saftey.
I'll be testing this a bit, but it seems to me that (depending on your play style and tactics), it might be better to invest in range boosts and maiming (slower target = more time to shoot from safety). It's interesting to note that only 3 DG naturally Ranged Attack (R, TB, QoT [only general]). Of these, afaik, Regulus is the only one who can increase his range...
TL;DR? - ROOK SMASH!