Re: bigger vs wider
One of the great things about this forum is the wide field of experience to draw upon for advice. One of the downsides is that those advisers inadvertently muddy the waters by tossing out their favourites as designs to be considered. Some feel it necessary to denigrate your choices along the way, and that is both impolite and sad. Read them all, then separate the wheat from the chaff as it pertains to your ambition, and carry on.
DGentry's comment about building the dinghy first is good advice that I would urge you to follow. The cost in time and materials is not excessive and the lessons to be learned are valuable. If it goes well and you retain your confidence in your skills after it is finished, I think that you can step up to the larger boat directly without an incremental approach, but that is just my opinion. If you are not satisfied with the results, build a small open sailing dinghy to hone your skills - again, not excessive in time & cost.
Now to answer your question: I think that the Bruce Roberts design will be the better-performing boat. The Glen-L design will be easier and faster to build and will be a better, roomier 'family-puttering-around' boat, but the Roberts design will probably be faster on the water and have an easier motion in a seaway. Their initial stability curves will probably be quite similar despite the difference in beam, but I expect the ultimate stability (the white-knuckle range of stability) of the Roberts design will be better than the Glen-L design. I also expect that the Roberts design will be capable of staying out in stronger winds and rougher waters than the Glen-L design. The downside of this encouragement to the Roberts design is that it will take longer to build. If there are published estimates of hours required to build the boat, as a first-time builder, double them. Stay focused and disciplined in your build efforts - it is estimated that 50% of all home builds longer than six months remain unfinished due to the builder losing interest in the project. And work safely - my usual urging to my clients (pro and amateur) is "make sure that you finish your day with as many fingers as you started with". Pay attention to the warnings about the chemicals such as epoxies that you use, too.
Good luck with your decision, and with the build of whichever you choose.
Hope for the best, but plan for the worst.