Casey Jones, Welcome to the wonderful world of garden railroading.
Flex track: Just a few of my thoughts.
1. Every rail joint has the potential of slipping apart a small bit thus allowing a wheel set to 'pick the joint' causing a derailment. Flex track being much longer than sectional track means MUCH fewer joints. Reduce the possibility of track sponsored derailments. I have used primarily LGB 1.5 meter flex rails, I soldered two sections together to make each section a full 3 meters in length. Made a 'jig' to hold the rails straight and true, cut a slot along the length of one side of a 2x4 just large enough to hold a rail with a 3 inch section removed to expose the rail joint for soldering.
2. Ease of shifting the track centerline. I have a 96 foot long "straight" run along the back fence. This run required dodging a few obstales, a grape arbor, a brick fence post, and two sprinkler heads. In the 15 feet between the grape arbor and fence post I needed to shift the centerline by 6 iches. Easily done by gently tugging and pushing on the rails. Much smoother than could be done with sectional track.
3. Non-standard curve size. Sectional track gives a limited number of curve sizes. Need a curve inbetween these sizes, no problem with flex track. Need a larger curve, also no problem.
Next question: What type material? Stainless Steel? Brass? Aluminum? Plactic? PLEASE remember that the folloing is my opinion based on my experience and likes, NOT a statement of absolute fact.
Out of the box plastic has a certain eye appeal realistic look. It limits you to battery power only, thus some of the very small items like a hand car will not be usable.
Aluminum is a good choice for pushing electrons down the rails, but corrodes rather quickly requires continious cleaning.
Brass is a better choice. It oxidizes at a realivity slow rate, I use a pole sander with a green scotch brite pad three time a year. It takes about 15 minutes each time for my ~800 ft of track. There are different brands and they are all different in composition of materials. Each will oxidze at different rates.
Stainless Steel. A good option for pushing electrons and it is oxidation free. The draw back for me, this is a personal opinion ONLY, is the shiny silver streaks running around the yard.
What ever make/brand you use, you will need to do a quick scan prior to running your trains for natural obsticals. In my case for 'melting' rose petals and dog deposits. Wilting rose petals block the flow of electrons and will gum up wheel sets. Dog (or other wildlife) deposits will cause colision derailments. Tree leaves and twigs will also cause problems.
Feel free to ask any questions/problems you may have, whatever the issue some body here has faced and solved it.