First off, the biggest differance between keeping softshell turtles from species such as red eared sliders is that softshell turtles are fully aquatic and don't often bask out of the water. In fact, in nature, the only time they really leave the water at all is to nest. Keeping this in mind, softshell turtles can be kept alot like fish.
Hatchling can be kept in a 10 gal with a substrate of 2-3" of sand. The 10 gal should be filled only about a 1/3 of the way full. Use a submersible heater to keep the water temp between 70 and 80 degrees. To clean the tank simply remove the turtle, carry the tank outside to a hose, dump out as much of the water as you can without loosing any sand and then rinse out the sand freeing all the waste trapped underneath it and then dump out that dirty water and continue doing this until the sand is clean. When you do this it's going to be hard not to lose some of sand but just try your best, you can always add more. The tank
needs to be cleaned at least once a week. Softshell turtles are very sensitive to to dirty water and will develop sores and bacterial infections if their tank is not kept clean.
Just because they don't bask out of the water like other turtles doesn't mean they don't bask. In the wild, Instead of basking out of the water they bask in the shallows with their bodies concealed in the mud. With their long snorkle like noses they are able to simply extend their neck to breath instead of coming to the surface. Knowing this, please keep in mind that softshells still require the same amount of light with the proper levels of UVA and UVB that all turtles must have in order to properly grow, develop their shells, and absorb calcium. For your hatchling, you can light it's 10 gal with either a 20" strip light and 18" Zoo Med Repti Sun 10.0 flourecent bulb or heat lamp and 100 watt UVB Powersun heat bulb. The heat lamp and Powersun bulb is the best way to go but is also the most expensive since those bulbs cost between like 40$ and 50$. Either way, you can keep your softshell in a setup like this until it out grows the 10 gal which is around 5" or 6". At this size you should move it to a new tank between 20 and 55 gal where it can live out the rest of it's life.
This bigger tank should have 4-5" of sand and can be filled almost all of the way allowing you to use a power filter which means no more weekly tank cleaning!
Don't fill the tank all of the way because you want to leave some room for your turtle to stick his head out of the water to breath and look around. Leave about 2-3" of glass at the top of the tank. Also, to really make sure the tank stays clean use double the filteration you normally would for a fishtank that size. In other words, if you have a 55 gal tank, have a filter or filters that pump at least 400 gph. You can also buy some bottom feeding fish like some of the larger species of catfish to eat any leftover food scrapes. Lastly, now that you have a bigger tank you'll obvi need a bigger light. Again, you can buy another strip light and powerful UVB emitting bulb, or you can just buy another Powersun bulb going up a watt size or two. And don't forget a new submersible heater to handle the next tank size up.
Oops I almost forgot, FEEDING. As with any animal you want to vary your turtle's diet as much as possible. You should feed it commercial turtle pellets, small minnows like red tuffy's, freeze dried krill, crickets, earthworms and other meaty foods. Just remember, when it's a hatchling only feed it foods that are less than half the length of the turtles shell. Depending on how fast you want your turtle to grow, you can feed it daily or every couple days.
Last but not least, to answer your first question "whether you should keep a western spiny or midland smooth" is completely personal preference. You should look at pics on google and see which one appeals more to you. All softshell turtle care is pretty much the same .
Good Luck!
