Because you aren't paying for dual suspension you can make your dollar go further with a hardtail vs. a full suspension.
Hardtails help you learn technique better because you have more feedback from the trail under you. You learn how to separate your body from your bike a lot quicker on a hardtail than a full suspension. Hardtails also have you work on skill and body positioning when it comes to climbing.
A full suspension can "squish" and give you added traction when riding uphill. A hardtail really has you learn the basics.
I recently wrote a review on my Specialized S-Works Epic Hardtail, which you can read here.
I think what helped me appreciate the hardtail more for what it allows you to do (or doesn't help you with) was my experience on other bikes. Gaining confidence in my handling and control- figuring out how to better move my body for certain things. It really let me see how my skills had progressed and allowed me to further work on bettering them.
Carbon vs. Aluminum...
*Carbon is going to be lightweight and can dampen trail feedback/vibration. It's not going to be as active as suspension, but it can make quite the difference.
*If you live in an area where you'll do a lot of climbing you might find going with as light of a bike as possible to be helpful. You'll still gain fitness and have a workout, but you aren't pushing a heavy bike up a hill.
*If you want the bike to be multi-purpose for gravel, pavement, and mountain biking- going carbon might be a good choice. It's lightweight and again, carbon material dampens vibrations, so can make the ride less fatiguing.
*Carbon is spendy but can also be repaired.
*You will spend more money on carbon. So you should do a comparison between a carbon and aluminum bike and their component spec, to see which is going to benefit you the most.
You can spend more money on a bike with a carbon frame with lesser quality components vs. spending the same on an aluminum frame with upgraded components. It will come down to what you feel will be the best value vs. what you may want to upgrade in the future. Is getting a lightweight frame that you can upgrade down the line more important to you vs. the drivetrain and other mechanical bits? You can lighten up an aluminum bike with carbon bits/wheels, etc.
Aluminum is going to be more budget friendly.
There is a lot to think about when investing money into a new bike and this blog post isn't going to give you a black and white answer because every person will have different wants/needs. It will take research, asking questions, and riding bikes to figure out what is going to be the best route for you.
Wheel size is another rabbit hole that can be confusing and depending on our height and the brand, your options may be limited. My review of wheel size is strictly my experience between the two brands that Decorah Bicycles sells: Trek & Specialized.
At roughly 5'2" Trek would put me on a 27.5" wheel and with Specialized, I'm able to purchase a full suspension or hardtail with 29" wheels.
Do I feel 29" wheels are "too big" for me at my height? Most of the time I would say no. I personally like to have an option, and I do feel that the 29" wheel size is best for what I want my hardtail to do, which is be multi-purpose. (Gravel/Road/MTB)
I feel 27.5" wheels are a fun wheel size to ride on singletrack, and my Procaliber 9.8 felt quick to accelerate and going into turns felt quick and nimble. Folks find 27.5" to be more a more "playful" wheel size where 29" wheels lend to stability. 29" wheels are going to be the faster wheel size, but that shouldn't be the only thing to base your decision on.
I would recommend trying out a hardtail in both wheel sizes to see what size you feel works the best for your riding. I would suggest going with the wheel size that will best meet your needs for what you'll primarily use the bike for. If you're mostly mountain biking and looking to do the occasional gravel ride the wheel size might be different than if you're looking to use the bike for gravel riding and the occasional singletrack rip.
A lot of folks have found a hardtail to be a great option for mountain biking in Decorah or anywhere else that could be considered "cross country" riding. Overall, it's a bike that will help you learn how to use technique to achieve your end result, like climbing up a hill. In my opinion, a hardtail is a great bike that will help you grow and develop good handling skills that will take less of a toll on your body than a fully rigid bike.
Is it the right bike for you? You'll have to ride a few and find out! Remember to keep in mind of future needs rather than solely focusing on current needs to help you make the best choice possible.