I bloom under a 4ft 4bulb t5 with 6500k bulbs from time to time. Yields are nothing to brag about but better then not using the light at the moment. Its gets big biddah cheese nice and frosty.
Use the Metal Halide, man. You'd be getting way more lumens per watt. But, if you can, toss some 2700k bulbs in that T5 fixture to add to the MH for flowering. That's what I would do.