I've never personally had any issues with accessing my snakes enclosures from above. Like said above, a snake will defend itself from any angle if it feels threatened.

As far as glass goes, I currently have 4 snakes all in glass and am going to be switching over to PVC over time as funds allow. The humidity is the biggest issue, especially during the winter months. That's not to say that glass is no good, it's just a little more work. You can wrap three sides of the tank in anything from paper to decorative terrarium backers to make the snake feel more secure, eliminating that issue. One thing some people, myself included, like about glass is the display aspect.

As far as a BP vs a corn, they are both the most common snakes for beginners for several reasons. I have both as well as a King. They are all amazing animals and I enjoy keeping both colubrids and BPs because of their differences. You can certainly get a visually appealing snake regardless of which you choose, so it's going to come down to what you're looking for as far as temperament, handling, size, etc... Corns are much faster, tend to be much more curious while being handled and are almost always moving and exploring while you have them out of their enclosures. BPs are also going to be curious and want to explore, but tend to be willing to just sit and chill with you more than a corn would.
The size is another difference, as you already mentioned, but also a matter of personal preference. Both are very manageable size which works well for most people, especially new snake keepers. I will say this, corns can be very fast, and because of their slender bodies, can be incredible escape artists. Regardless of which you choose, you will need to make sure the enclosure in escape-proof. Corns can also escape your hands if you give them a split second. BPs are slow by comparison and their thicker bodies make them easier to handle, in my opinion. This may seem like I'm trying to sway you, please don't take it like that. I have 2 corns and only 1 BP (for now ), as well as a King, so obviously I love the colubrids and plan to add more over time.

Regardless of which you choose, choose the right snake for you, after all, you're making a commitment to the animal for years to come, so it really comes down to what YOU want. I definitely think you will enjoy your snake whichever you choose. And if you're like most of us, you won't stop at one snake, hahhahaha.

Don't be afraid to ask questions, that's what this forum is for. You're in good hands here, lots of great, experienced, knowledgeable people willing to share their passion and help out.
Good luck!!