Mephrit Rapid-Fire - 4 shots > 2.667 hits > 1.77 wounds > 1.18 vs 2+, 1.48 vs 3+, 1.77 vs 4+ armor, 0.88 vs 4++
Mephrit MWBD - 4 shots > 3.33 hits > 2.22 wounds > 1.48 vs 2+, 1.85 vs 3+, 2.22 vs 4+, 1.11 vs 4++
What does the math tell us?
The particle beamer's got volume of fire over the other weapons. This gives you two things - first, you're more reliable just because you throw more dice; second, you get more wounds and force more saves. The twin gauss is technically a little more effective, but really pulls ahead when you go Mephrit and go My Will Be Done into 2+ armor.
So, let's talk about range - first off, you're on a platform with a 14" move and the FLY keyword. Odds are good you can get into the range you want, unless you're digging a unit out of the corner. So, let's look at the weapons.
Particle beamer - it's the shortest range at 18", but you're at full effect at 18 inches. That gets you charged by jump troops, but foot troops are a bit far unless they can advance and charge.
The twin tesla carbine is full effectiveness at 24" - you're arguably the least effective gun, but you're definitely safe - very little in the game gets to you from 24" away in assault.
The twin gauss blasters really need to leverage rapid fire to do the work - having a 30" range is neat, but rapid fire is the difference between 'incidental gunfire' and 'closed-casket funeral' - 15" is definitely in range to get tagged if someone wants to make it happen.
There are two base stratagems Tomb Blades care about, plus the Mephrit-unique one. Tomb Blades can pay a CP to turn rapid-fire weapons into assault 2, or fire assault weapons without penalty - obviously this is a little better for the tesla and particle beamer.
The other stratagem is just good for necrons in general - Disintegration Capacitors say we auto-wound on 6's to hit. This again favors volume-of-fire weapons, so the assault weapons pull ahead here.
Finally, we've got Mephrit's Talent for Annihilation - the first three 6's to wound do mortal wounds in addition to normal wounds. Again, we like volume here - so once again, the assault weapons pull ahead here.
It is worth noting that you could take a single tesla carbine for malevolent arcing - you just need one tesla weapon to target something. For 5pts, it's an option to get mortal wounds.
You start with a 25pt model - you can go up to 30 if you upgrade from the particle beamer to either option. You can proceed to dump plenty more points into these guys, but I question whether you'd do that. Defensively, you are a T5, 2W, 4+ model that takes a -1 to hit vs. ranged combat.
You can pay 3pts to go up to a 3+ save, or buy a 5++ save for 5pts. You can also pay 3pts to ignore cover. You could even, if you wanted to, mix and match the saves. I think I'd rather get the 5++ or go without - volumes of AP-1 and AP-2 aren't hard to find, but 5++ will at least slow down higher-damage weapons.
I'd consider the nebuloscope's perk for tesla - yes, particle beamers like it as well, but the point of the particle beamer is cost. Nebuloscope's take that away.
Keep It Cheap - particle beamers, nothing else - take 6-9 for 150-225pts in a unit, throw 36-54 dice at stuff, laugh. Run mephrit.
Some Quality - Buy gauss, shadowlooms - 33pts a model, pay 198 for 6ea. You're reasonably durable, you don't care if the inevitable rapid-fire fun coming back is ugly.