It really depends on your skin type whether you are oily, dry or combination and the amount of coverage your looking for:
if your oily - your best to go for mineral powder foundation although you can go for liquid's especially because mineral foundation is much less likley to block your pores because its ingredients are natural but if you go for liquid and have oily skin its best to go for a liquid thats consistency isnt too runny
if your dry - your probably best to go for a liquid mineral foundation because it will add moisture to your skin but you have to also watch that it doesnt sink into your pores and dry the skin out and if you have dry skin you probably want a liquid that is quite runny alternatively you could use powder also if you have dry skin but you should use a moisturizer before hand that is suited to your skin let that sink in then apply your powder
if you have combination skin - either one is suited to you and you can go for whatever one you prefer though again you should use a moisturizer for all skin types that is suited to your skin let it dry and then apply your make up.
You asked if mineral foundation is better than liquid but mineral foundation comes in both forms liquid and powder i havent yet seen a creme mineral foundation but i am sure they are out there but mineral foundation is better for your skin than normal foundation as it doesnt have the impurites in it and its ingredients should be a 100% natural which means its less likley to break you out etc.
although beware! because some cheaper brands such as drugstore brands claim to be mineral but they might not be a 100% mineral although they will have minerals in them
Probably one of the best (and one of the first) mineral foundations is BARE ESENTUALS and they do many other products such as mineral eyeshadows as well as foundations they are however a lot higher in price than your drug store brand at £22 from John Lewis but you can also get them from qvc and i am sure debenhams and of course they're own website sell them who currently have 15% off i believe and although they are pricey they are 100% minerals. Their foundations come in the form of powder and in deals you usually can get a free special brush for applying the foundation and it also has SPF15.
One the cheaper side you can get:
Maybelline New York Pure Mineral Foundation for £7.99 it is liquid and i have heard it gives a light coverage they also do a concealer that is mineral for £5.99 you can also get its foundation in a powder form for £9.78 and with a free brush i believe
Collection 2000 (which you can find in boots) also do they're own version for £4.99 but to be perfectly honest i can guarantee that all of its ingredients ae probably not 100% mineral it is in powder form and claims to be a foundation, sun screen and concealer all in one.
L'oreal also do one called L'oreal Paris True Match Minerals its powder and is £13.99
Rimmel also do one its liquid and called Rimmel Lasting Finish Mineral Foundation £5.99
Really all brands do have a mineral foundation i could go on for days really what your best to do is have a look at a brand you like and see if they have a mineral foundation which they probably will
you could try that or you could have a look at several brands you like and look at reviews of them on the internet just type in google "products name" reviews or people post videos reviews on youtube
and look at what peopl are saying about its coverage what age they are they're skin type and relate that to what you want and who you are. To be honest if your really wanting the Best Mineral Foundation you probably would get the best out of a brand such as Bare Esentuals to get the best but i can understand that you aren't wanting to pay a fortune, really in the end you might find that your best suited to just a normal foundation but i am sure with some research and testing and trying you will find out because really people can tell you what suits them but you never know what suits you till you try yourself
sorry for the length of this but i hope it helps! :)