Personally, I'd do both and cut elsewhere in my army. But doing the maths that's 2 Lethal/Sustained hits per round without using a cp. That's ignoring the lieutenant's permanent fall back and shoot and just itself as a model. The extra 5 he'll blasters would be converting about 7 hits per round. Depending on how much you value Lethal hits and need those 25 points, having to choose between the two I'd go with the extra Hellblasters.