Solder not wetting the surface of the tip indicates an incompatibility between the solder and the tip material. This can happen when the external plating on the tip wears away or is contaminated.
You don't mention what you are soldering, so how many Watts you need can't be determined.
I have not used Antex irons but after a quick search it seems they are at the lower-end of the quality spectrum. If you are doing a lot of soldering then a better iron with replaceable tips will serve you well.
For surface mount work I use Weller, Metcal and Hako at work and all are good. Weller is the most affordable of these but, again, the tips oxidize/wear out most quickly of the three. All of these have replaceable tips ranging from $10 to $75 per tip. The Metcal is ten years old, the Wellers are eight years and the Hako is two. We keep a small variety of favorite tips and replace them as needed.
- Make sure your solder has a rosin core; without rosin in the solder even leaded solder is difficult.
- Make sure your solder does not have an acid core; this stuff is only for plumbers with torches.
- Clean the tip only with a slightly damp cellulose (not metal) sponge. I never use anything else. A quick wipe is enough to remove the excess solder and contaminants from the last soldering operation. Note that a very wet sponge will cool the tip quickly which could contribute to a shorter life.
- If a tip cleaner/tinner compound improves the tip performance it really means that the tip is worn out and needs replacement.
- The tip may be too hot and oxidizing the plating. If it is adjustable, try turning it down.
- The tip may be too cold and requiring too much time to heat the work, resulting in extra wear and tear.
- If the iron is inexpensive, the tips may not be built to last. Try a more expensive iron. There is a lot of technology in making a tip plating that will last and the quality vendors don't give it away cheap.
- Return the iron to the holder with a drop of solder on the tip. The solder will help protect the tip from oxidation. Wipe the excess solder off of the tip just before use.
- Turn the iron off when not in use for more than 20 minutes or so.
- Use the tip against the work with care. Rubbing, scratching or scraping the tip against the surface being soldered will wear it out. If you always hold the iron the same way, you'll wear out one spot quickly. That worn out spot quickly oxidizes and undermines the plating on the remaining parts of the tip.
- Make sure that only solder touches the tip. I've used irons to make holes in plastic and other such unorthodox uses, but the tip suffers.