Answers

Hi.  Hardi is a siding made of painted cement board.  SmartSide is a painted wood product.  Both are rigid materials and stay perfectly straight on the house.  Their drawbacks are very visible seams and both require painting after about 10 years.

Based on the information found via the internet, I believe the difference in siding materials depends a lot on where you live and what kind of climate you experience year round for your location. I'm including a link here that shows what one inventive father did with his children to promote education in understanding their climate of California. It's a very good way of finding out the semantics of a product while spending quality time bonding with your children. Best of all, he's done the difficult work of figuring out which materials stand up to severe climate changes for you!

http://www.3ghomeimprovements.com/2017/06/james-hardie-fiber-cement-vs-lp-smartside-vs-insulated-vinyl-siding/

Hardi™ is a fibre-cement product and the LP product is a chip-board/resin composite. The Hardi™ comes both finished and unfinished, the LP is unfinished and must be painted with a high quality exterior coating that meets the demands and longevity requirements of the region in which it is utilized. Finished Product warranties vary based on the above variables.

LP is a woodbased product, where Hardi Plank is a fiber cement. They ar both very similar in cost here in the midwest. LP Smartsiding if fairly new the market as well. 

Hardi has about 30% cellulose fiber in it and the rest is cement. Hardi shrinks after it is installed so you must keep it tight to the the next board and not leave expansion gaps on either side. It definitely must have slip sheets under the joints. It must be caulked at the corner posts and openings. If you  cut it, it must be doped or painted as it is installed.

LP Smartside on the other hand expands after it is installed. Approximately 3/8 of an inch. horizontally for the length of each board. They have a gauge to place between each horizontal lap to allow for expansion. Here again I would use a slip sheet behnd the lap. Both prodcts are susceptible to moisture. So here again you must dope or paint any cuts as you install if you are using a predinished product. Both Hardi and LP require you to where masks to keep from breathing in the dust. Not good for your lungs.  

If you really want a good product with a superior finish, consider steel siding. Vinyl is just plastic and it breaks when hit by hail requiring it be replaced. Get a grill any where close to it when barbeciung and it melts.  Steel can dent in a major hailstorm, and it normally doesn't. but it still won't leak. Hardi board and LP will cost you more to install if prefinished than steel and  steel is virtually maintenance free. My house has steel on it since 1975. Most vinyls will last unitil the next big hailstorm and then you have to replace. The one drawback for steel is, it is harder to repair than vinyl. but can be done by a skilled craftsmen, without removing the whole wall. 

HardiePlank is a fiber-cement siding, whereas SmartSide is an engineered wood product.  Fiber cement products do not rot as easily as wood products and most builders will recommend fiber cement over wood for that reason.  If you choose either of these products, you will need to be dilligent about maintaining them (painting, checking for rot, etc.).  Your other major option would be vinyl siding, which does not have the aesthetic appeal of wood or fiber cement, but is much easier to maintain.