For the first post of the year, I will bring up a question that has plagued gamers since the beginning of 5th edition. To mech or not to mech? That is the question.
With the start of 5th edition, there was one major change that stood out more than any other. That change was the increased survivability of vehicles. No longer could a standard vehicle be destroyed by a glancing hit, and on a penetrating hit, the chance of destruction was brought from 50% down to 33%. Games Workshop didn't stop there. In addition to tanks being more survivable, almost every standard transport was reduced in cost when their new codex dropped.
This got people thinking about how metal boxes would insure that their dudes wouldn't die so fast. It also helped assault armies get across the table more quickly. Eventually, pretty much every army, barring tyranids, grey knights, daemons and necrons, was zipping across the table in their own APC.
The metagame, however, would not allow this to continue forever. As more and more tanks came into play, more and more anti-tank weapons came into play. At a tournament scene, a player could expect to come up against mech, and would pack as many lascannons, meltas and missle launchers into their list. With this, mechanized armies died down slightly. While these lists are still quite popular, lists made up of entirely infantry (simple lasagna, man!) sprang up to contest all of these anti-tank lists. Now a line is drawn, and many people are stuck in the middle, unable to decide.
I will not say which is better, for I don't believe that either one is. It depends on your army, and/or the way you want to use that army. Mechanized and foot lists both have pros and cons, but most armies are geared to be able to competitively (for the most part) use either.
With that, I will go over both armies I own and sum up why I play them the way I do. Hopefully I can give a good example that way, without imperatively saying which is better.
My first army, and my most tested. I run them mechanized, very mechanized. I run them so mechanized that I normally don't put a single infantry model on the table. The reason I run them like this is first off, because they have great transports. 55 points is giving me a 12 10 10 transport with 2 heavy weapons. With that, I.G. also have amazing battle tanks that never make me sad. The reason I love tanks so much is that they protect my infantry and form walls. Imperial guard are good at shooting, not staying alive. Now, this will come up later, but I've noticed that the only armies that should take transports are the armies blessed with cost-efficient transports. I.G. is one of those armies. I have played with platoons a lot, especially back when I had a single chimera, and they definitely have their place in the game. I would never want to imply that infantry guard lists are bad in any way, but they didn't impress me half as much as fully mechanized. I have also toyed with hybrid lists, or a little bit of each. I found a good balance here, but found that I'd rather have small arms fire and anti-infantry guns patter off of my armored chimera hulls than kill my dudes. In my current 1,750 I.G. list, 1,005 points are spent on vehicles, and I have 75 models.
I run my eldar fully on foot. Not a single tank in my 1000 point list (unless you count war walkers). I once ran my eldar with 3 wave serpents and a fire prism, but my ways have changed to that of simple lasagna. Truth is, the cheapest I could possibly get a single wave serpent was 100 points. As far as firepower goes, the wave serpent has less than the 55 point chimera. As far as survivability goes, it is a 12 12 10 vehicle with resistance to S9+ and weapons that roll 2d6 to penetrate. As far as cost-efficiency goes, it is much worse than the chimera. A single krak missle has the same chance of destroying it as it does a chimera when going against front armor. Yes, the wave serpent is fast, but when it comes down to it, the metagame supports multiple cheap units to single large units. Eldar are not blessed with amazingly cost efficient transports, which is why I don't prefer them to full infantry. I would like to bring up that my eldar are doing as well as my guard on the tables, if not, better. This is partially due to the smart use of cover, and I doubt that someone would consider the armies to be on par with each other when it comes to scariness. In my 1,500 point eldar list, I spend 0 points on tanks, and have 90 models.
With this, I still don't believe it matters a lot. I know, not much of an ending, but I say play what you enjoy. If you like parking lots, play mech. If you like tons of little dudes, play infantry. If Games Workshop is doing anything right, it shouldn't matter in the tournament scene.