I made my living as a personal trainer for several years, and I was always at the gym early to train clients who wanted to workout before starting their workday. What I noticed after a while was that those people who consumed a moderate breakfast and allowed reasonable time for digestion before they trained had better workouts than those people who reported that they ate nothing. "Better workouts" meaning that they warmed up faster, exerted themselves harder, and made faster progress towards their goals.
Also, I began to find that I could tell the difference between when one particular client, who ate breakfast only sporadically, ate his breakfast and when he skipped; I even had him record his meals on paper, without telling me when he ate a breakfast and when he didn't, while I recorded his rep ranges and poundages (without telling him I was doing so). After a few months I compared the two log sheets and found that they showed better performance on his part when he ate breakfast. He ate clean.
On the other hand, I had clients who stuffed themselves with Denny's Grand Slams and stuff of that ilk right before showing up for training. These people generally warmed up very slowly and performed poorly right up until the end of their workouts, when they barely started to show signs of life.
So based on that experience, and my own BB experience, I'd say that what you do before a workout can best be determined by keeping a detailed log, including meal times and quantities, along with a notation for your energy level and the quality of your workout. Just give your workout something like a five star rating system. If you eat a plate of Aunt Jemima pancakes before reporting for gym duty and you have a two star workout, whereas you enjoy four or five stars with whole grains and Greek yogurt, then your better option is clear.
As far as gains go, whether or not you add more muscle mass with a pre workout shake (assuming it's a quality shake) or a pre workout meal can be hard to determine, except in the long term, and with all other variables held constant. I've lifted with some serious guys with high quality physiques and I've never known anyone who made an effort to determine such a thing one way or the other.
However, it's my opinion that you don't have to determine such a thing, because what's important is keeping track of the quality of your workouts. The rationale is that, in the long term, your gains will be more significant if what you do pre-workout is predicated on your performance in the gym, under the barbell. Motivation and enthusiasm will be higher if you can consistently look forward to a great session.
