It all depends on how good the concrete is - both the surface condition
and the structural condition. If the surface is loose, forms dust or
crumbles, then nothing can stick to it so tiles and paint are impossible
and a liner is required. If the pool concrete has cracks in it, which can
occur for many reasons, then it is probably moving and the cracks are
expanding and contracting with either soil movement, soil water content
changes (drying or saturating) or foundation failure. Tiles would crack
and the pool would leak, same for paint. So you need to remove the liner,
let it dry out and check the structural and surface conditions before you
do anything.

If the concrete surface is poor but not cracked, or at least not cracked
too badly, then a liner would likely be OK because it doesn't need to
stick and can move or stretch a bit if the pool walls move.

If the surfaces are good and there are no cracks then you can do as you
please and can afford. Tiles are the most expensive by far and totally
dependent on quality of installation - get the best guys in town and get
a long warranty. There are a number of ways to really screw up underwater
concrete work so experience counts. If the pool is not heated in the
winter and the walls above the waterline can freeze then tiles may pop
off due to freezing action even for a good job.

The least expensive method is to paint it but research the pool paints
carefully, many only last 5 years at best and need to be repeated that
often or even more often. Make sure you use the proper primers first -
sometimes there is a pre-primer required for a particular manufacturer's
product. All primers and paint from the same manufacturer and applied
exactly as required by manufacturer. If the painter says he never does it
that way or it is not necessary to do it that way then get another
painter. The first guy won't do it when you are not looking. Make sure
contract states that paint must be applied according to manufacturer's
instructions and don't pay if it is not. Avoid white color as most of
these turn yellowish in patches. Make sure top coat paint is rated for UV
protection and use in pools with chlorine or salt in them. Realize that
there is in fact no quarantee with a pool paint job. If there are
problems later the manufacturer will claim that surface preparation or
concrete conditions are to blame. The painter will claim product was at
fault or concrete conditions are to blame. You will have no way to know
which is true and no recourse at all. So paint is a risk. It is cheapest
but there are no free rides.

If the concrete surface is uneven it can be smoothed out, not perfectly
but pretty well, using Fast Set. Make sure there is no water used to mix
up the Fast Set, just acrylic admixture. The admixture makes it stick
better, helps for underwater conditions and helps resist frost problems.
No quaranty but it improves chances hugely. Without admix it is all a
waste of time and money.

Note that the pool must be perfectly dry for tiling or painting. You must
keep rainwater out. I'd use a tent.

You could look at replacement of the entire pool as some posters have
suggested, especially if the concrete has a crumbly surface or has a lot
of cracks or just a few big cracks. Cheap repairs of poor concrete are an
endless frustration and hugely expensive both first time and then get
more costly repairing continuing problems. It never ends. Throw it out
and start over.

Advantage of replacement is you can get a warranty from the pool company
for all the bits. A warranty is only as good as the company backing it so
avoid the cheap guys - problems always occur so the company must charge
enough to be able to afford to keep it's customers' pools in good
condition, i.e. you must pay for the warranty. The cheap guys usually
don't do any or not very good warranty work, that's why they are cheap.

I've found huge differences in cost of concrete demolition - variances of
200% or 300% and often the best companies are the least expensive. So get
at least three quotes and better yet, four. Tell the companies that you
are getting several quotes so they know it is competitive. Pay extra for
the experienced company, if the costs are close. The largest and best
demo company in my city gave me a quote to demolish and haul away a 2,200
square foot house, including the concrete basement, that was a third of
the highest quote and half of the second lowest. They did a great job.