They are very similar and I have actually used both on one project. Latex does seem to be a bit thicker(more like poster paints as opposed to acrylics. I suppose it is to provide greater coverage on a large flat suface. I use a paint conditioner to thin them a little, especially to allow carved or built up details to come through. Using water as a thinner doesn't seem to work as well for me, as it dulls the colors. The paint conditioners thin the body of the paint nicely.
If you are planning to use a varnish or other sealant, I would definately use flat paint. The glossier versions don't accept sealers or even other layers of colors as well, since they don't have a "tooth" to accept them. Does that make any sense? I'm on my first cup of coffee LOL.

As Papersoup said, latex paint tends to be a bit thicker than acrylic.  I've mixed acrylic and latex paints together numerous times without any problems.  Another positive aspect of latex paint is it has some flexibility to it, so it's less prone to cracking--you can even paint bendable wire with it and it'll usually remain intact when you bend the wire itself (provided the bend isn't too extreme).  The only negative aspect I've ever noticed with latex paint is that the glossy ones can be kind of tacky/sticky when they're dry, even years later.  I'm thinking of one figure in particular that I painted with latex whose feet always want to adhere a bit to the shelf I keep him on.

Walmart, assuming you have one nearby, sells Folkart/Apple Barrel acrylic paints at fairly reasonable prices if Michael's costs too much (about $2 for an 8 fluid ounce bottle of matte, gloss costs more). That's where I get all my acrylic paint.  But, yeah, if you're going to be making large projects, a can of paint would probably be a lot more economical--heck, you can probably find friends/relatives that will be happy to give you old cans of paint just to be rid of them.

I don't know about the Great White North, but here in Texas our recycling centers take in leftover paint and readily give it away for the asking. They have a couple of pallets of paint cans at my local center that you can just pick through and take what you like. I've picked up a lot that way.
LOL I've always been the ultimate cheapskate, but now it's called "environmentally friendly"

Joey

I've never heard of rain making paint tacky, but I suppose it's a possibility.  I know I painted the figure indoors, but it was made years ago, so I couldn't say if it was raining or not at the time.  I've got other figures that are painted solely in gloss latex that also have a tendency to stick to shelves, so I'd have to lean towards it being the paint, rather than environmental factors.

I am going to experiment with latex housepaint soon (mostly as a sealer, since so many seem to use it), so I cannot be sure how appropriate it is as a paint for paper mache.

However, I worked a lot with those little bottles of craft acrylics, and can tell you they are usually robbing you of money. At the price of a bottle, seeing how little they cover, and seeing how brittle they are compared to the good stuff, i,d say it's a scam.  Seems like a bargain because of the price tag per bottle, but it adds up quickly.

I highly recommend switching to artist quality acrylic paints, either in tubes, in jars, or make your own from the base ingredients to have full control of all properties, and save money at the same time.

I mostly make my own acrylics from dry pigments and gloss acrylic medium. Adding more pigments makes the paint perfectly matte. Talcum powder is also a good matteifying agent.  Artist Quality acrylic paints are supposed to be rated for lightfastness, be well saturated, and have good adhesive and stretching properties.

Getting my ingredients from a specialized pigment store aves me a lot of money.
Here is my supplier, who also deliver outside the country. If you don't have a pigment supplier locally, I'd recomend getting the dry pigments from them, but try to locate the acrylic medium in a local art store, because liquids are heavy and that could increase the shipping costs.
http://www.kamapigment.com/index_en.html

Have fun!

Jackie:  "...it's practically impossible to make the exact same shade again!"

Aack!  There's no 'practically' about it!

But there is one trick which can be used in a few circumstances. 

If you have to mix more of the same color and you think you're close, use it on a different plane.  For instance, if you are making a dollhouse of PM and you're running low on your wall color, use what you've got left to finish as many full walls that you can.  Then use the almost-right color on adjoining wall.  The difference in the light hitting the difference planes can offset the faint difference in color.

The other trick is separating the faint difference with another color, to trick the eye.

Sue

Otherwise, you have to recoat the whole section.