I like both. T5's and T8's are a good choice for indoor growing. Depending on how many bulbs are being used (along with wattage and bulb length), either one should provide adequate lighting for most plants, especially if the fixture has good reflectors.

In regards to "preference", I use t5ho (ho = high output) because I already had the bulbs & fixtures on hand (didn't have to reach into my pockets to buy and supply lighting for my plants). If I were starting from scratch, I'd probably go with t8's as my preference... a bit less expensive than t5ho's, and easier to acquire (bulbs/fixtures) locally.

As to your question: "can you put a T5 into a standard shop light fixture?" - depends on what you mean by standard shop light fixture. I've noticed several "shop light fixtures" in some of the big box stores recently - T12 fixtures (T12's are starting to be phased out), T8 fixtures, and T5 fixtures.

Some of those fixtures were quite nice (quality ballasts and good reflectors). The number next to the letter "T" (e.g., T5) corresponds to the diameter of the bulb; the larger the number, the larger the diameter. You want to use the correct type of bulb in the fixture it was deigned for. A T12 isn't going to work in a T5 fixture (and vise-a-versa). Also, a high output bulb (ho) will not work properly in a normal output (no) fixture. Hope this helps somewhat regarding your question.

Last years lights I used T-12s
This year I made a set up of T-8s
The T-8s are dong very nicely for me, all I'm using are the daylight style.