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well technology for one, ballasts are different, the most common fixtures were t12s, then t8s, now t5s. They make other sizes, but the t5 is the most popular one at the moment. It produces much (especially the HO version) more light intensity than a similar sized t8 or t12.
Yes, T5, T8, etc are long skinny tubes. MH's are a bulb similar in appearance to what you'd see in a security spot light. Most MH fixtures are square-ish