There are a number of things one might consider when choosing the right marker, for example;
~ If you are a German defending the Atlantic Wall a nice electro would be a great choice, firepower via volume.
~ Maybe you're gunna be one of the Allied landing on the beach. Since there is water in the pond something Non-electro would be a good choice.
~ Rain is also another factor that could make choosing something mech over electro wise. But you can still take those electros out in the rain, I used my Bushmaster during the early morning rains of '05 with great success. Trash bags make wonderful ponchos for your marker....
~ Physical build and/or capabilities could be a consideration as well. You'll be lugging paint, air & water as well as a marker for possible hour(s). If you're on the smaller side think about using something light. In '04 I used a Trracer pump and made several eliminations, of course I was carrying a nerf launcher as well.
~ Plus there is style of play.... do you like to run & gun or fling paint like a tourney backman? If so you probably want something electro. Like to stalk and crawl about? Then something more durable like a 98C fits the bill.
There are many many more things that could impact a decision but I think you get the idea..... Now there is one thing that I always thought was an important consideration that a lot of players don't think about and that's trying to match your Marker/Air to the amount of paint you can (or want) to carry. No need to carry 1200 rounds onto the field if you can only shoot 700 before you need to re-air.