I prefer 175 Watt Halides with other supplemental lighting with them like T5's / PCs.
Or an ALL T5 set up if you like that, there are great color hues & Kelvin combinations.
But even on your present Halides- what's your Kelvin rating?
10K ? 14K ? 20K ?
14 or 20 are going to be a bit less bright even at the same wattage.
14K is a good middle ground daylight with still plenty of blue usually.
20K usually is quite a bit dimmer looking & more blue, (deeper water effect)
You may want to check that Kelvin.. ? If your 10K at 250W it may be a bit bright for your tastes.
(I didn't like 10K even with 175 watt.)
6-8 T5's will do just fine over the 125. (retro style)
Personally I run 175 Watt 14K MH's with 2 VHO's(1 actinic, 1 pink) and 2 super actinic T5's & with 1 double purple PC. Over my 125.
T5's are great lighting and I think you can keep anything with them alone.
As long as you got enough wattage. You don't "need" MH.
But I do prefer to still keep MH with mine, especially for the shimmer effect.
Depending on your reflectors (preferably singular for each T5 bulb) you can still get some shimmer from T5's. But NOT like the superb shimmer from MH.
Either way, unless your tank is much deeper than a 125 long is, you don't really need 250 W halides.
I would either get a 175 W Halide set with T5 sups.
Or go ALL T5's.