Is it better to use worker drone/scv/probe to activate a Xel'Naga Tower or an army unit?

A basic soldier unit is better than a worker, both for ramp scouting, tower holding, or simple patrolling. Early to midgame, you should be building workers as fast as you can, as they are the life-force of your army, and pretty much every strategy can be traced back to them (harass to decrease enemy workers, push to exploit an early expand lest the increase in enemy workers overwhelms you economically, etc). Army units are secondary and much more disposable, not to mention can defend the tower, as well as sneak a kill against unattended or weaker scouts (depends on the matchup). The listed cost of a worker is 50; unless you have maxed your minerals, which happens much after the struggle for towers, merely the time it takes for the worker to get to the tower would've netted you at around 50 more minerals if that worker had been mining instead. Add in the time spent there, and suddenly losing 2 or 3 50-mineral units to get a tower sounds like a trifle compared to losing a worker for the same purpose.

You need all the workers you can get. If your base isn't producing workers for even a second (time spent upgrading human bases excluded of course), or if you're sacrificing any of them before you have 30 on every base and 2-3 bases, you're probably doing something wrong. Even for the Zerg, who use the same queue for both warriors and workers, you're better off with the cheaper and more powerful zergling.

To illustrate, check this chart*, which states that a single worker earns you ~60 minerals per minute. Consider the average time spent on a tower, and remember that you'll have missed on enough money to expand as soon as your worker spends 5 minutes on the tower (travel time included):

*chart source is http://www.teamliquid.net/forum/viewmessage.php?topic_id=321242

As this is more of an opinionated question i shall give my experienced answer. Having any/all of the Xel'Naga Towers on a map is very important in keeping map control, map control is incredibly important when it comes to controlling/spying your opponents movements.

It is much more efficient to control a Xel'Nage Tower then to keep a small force of fast moving units constantly scouting the area. Although doing both will ensure your opponent isnt trying anything sneaky. I personally will send a Marine(of course a zealot or Zergling will work in substitute), as this allows a little defense on these strategic points of the map. I have watched as one of my marines has destroyed a couple units of a force moving across the map unattended, as well as unattended scouts.

Now obviously controlling these Xel'Naga Towers gives you a decent advantage over your opponent in concerns of scouting. Keeping this in mind this should not be your only methods of scouting, as your enemy can still surprise you and they often will try to. The only time I will use a worker to control a Xel'Nage Tower is when i am setting him aside for a secondary scouting mission close by.

Imho I prefer always go with a soldier rather than a drone.

The reason for this is that many players go with a drone so your soldier will kill it and remain having control of tower. Chances are that enemy player will send drone and forget about it, specially in 4vs4