I believe this difference means that a portal system would result in people bypassing terrain by default, while a summoning system would have people zipping past terrain mostly to serve the specific group goal of a quest or raid. I think this favors the summoning approach as that would produce less content bypassing, but I'm open to disagreement on that.
Both portals and summonses have some interesting possibilities as transportation systems. Assuming that you've designed the world properly and that there's some reasonable world-story behind it, there's no reason why you couldn't assign the teleporter function (in either "push" or "pull" mode) to any in-world object.
So you could set up a teleport pad beneath an obelisk in a city as a gate to another place, letting it activate every ten minutes or so to send whoever's on the pad at that time to some specific location in the world. (Pretty much standard behavior.) Or you could assign the same function to a portable teleport pad that you pull out of your inventory and drop on the ground. Such a pad might be set to activate automatically when someone steps on it... and what if such pads could be invisible? It ought to cost a mountain full of cash to keep such things running, but it would be an interesting twist on minefields. *evil grin*
Another way to apply the teleport function might be as a kind of last-resort device. Push the button (or however it's invoked), and any enemy within ten feet of you gets transported to a random location (not inside any object!) 100 feet away. If you're getting ganged up on and need to make a quick getaway, such an object could be quite valuable. (Do I even need to mention how insanely expensive something like this should be to construct?)
An attachable teleport capability could work in pull-mode as well, albeit a little differently. For example, you could set up a one-shot summons marker in your home town plaza. When activated either on some condition (low on health/mana) or by a device ("Beam me up, Scotty!"), you're pulled to that location. Another possibility would be to put summonses on several small objects which could be given to friends, and key them to a master device. Activate the master device, and no matter where they are, whoever's got one of the small objects gets pulled to the location of the master device. Great for pulling groups together very quickly.
You get the idea.
Finally, the issue of griefing should be mentioned. (I didn't earlier, but this might be a good time.)
Blocking other people from accessing things, and doing things to other people without their consent (even if that consent is only implicit), tend to be considered griefing and will really, really irritate players. If a portal system can be blocked, that might make IC sense and even contribute to interesting gameplay, but it's still going to make some people very unhappy. They're not going to care that defense of a transportation node is realistic; they're just going to be upset that they can't go where they want when they want. A "summons block" (like the "teleport block" in Steven Brust's great "Jhereg" novels) will have the same effect -- it might be a very cool ability, but it's really going to tick off some players.
That means a couple of things. First, it brings up the question of whether objects should be collidable. If a game's developers haven't started dealing with this yet, eventually they'll have to... and when they do, they'll find it's a huge headache.
Among the flags that need to exist for every object are these:
- Can the object exist in the world on its own (outside a player's inventory)?
- Can the object be placed by a player, or picked up and moved by a player?
- Is the object collidable to any degree?
Not letting any objects exist in the world that can at some time have been in a player's inventory makes the world feel insubstantial. If you can't leave some mark on the world, how is anyone to know you've ever existed? As for collidability, it just feels strange to walk or fly through some large, substantial-looking object.
The second issue is the question of consent. If you allow players to do things to other players without their consent (like teleport them), you're absolutely going to hear howls of outrage. "I was right in the middle of [X]!" But if you require consent for teleports, you not only lose a number of opportunities for fun gameplay mechanics, you have to build a consent GUI ("Do you agree to [Y]?" And when you do that, you risk enabling "pester griefing" as someone repeatedly causes a consent GUI to pop up on your display.
I tell you, the code for MMOGs of any kind would probably be about half its usual size and take half as long to create if developers didn't have to hammer out special logic to prevent the many forms of griefing. Sigh.
Developing a massively multiplayer game is definitely not all fun and games.