[TheCrimsomBlade]

Let me start with "how usefull is illusion"
I have found illusion to be one of my best tools. My character is a warrior first, but there are many times that going into a situation and chopping my way through just isn't called for. so I have found that (Invisibility) is great when you need to steel items right out from under someones nose and not kill everone in the area especially when you need to deal with the same person at a later time, or when the area guards are the ones you need to steel from.
The second most used illusion is (Paralize) this works great on smugglers that are larger than you when you are trying to loot their caves and you find yourself encountering more than one. Paralizing one while you fight the other may just save you life. I would think somebody playing a Mage would find this as a very powerful weapon. He could Paralize everone in the area and then cut their heart out and move on to the next.

Now for the first Question;
"I have a small question: Of Restoration and Alteration, which of these is easiest to simulate with potions/Alchemy?"
In both Morrowind & Tribunal; Potions More Important in Battle!

1st,Alteration is used so often that you will find Potions for Water Breathing, Swift Swim, Water Walking, Slowfall, and Levitate easyier to simulate with potions because you can control just how long you want these effects to last without using up all your Magicka, that is when needed for short term effects.

2nd, Restoration potions are a must at all times during the game such as the Potions, Restore Personality,Fatigue,Magicka,Luck, and the two most important in my opinion are Restore Strength and Restore Intelligence. If you don't have the last two on hand and it could destroy you in your tracks.

3rd, Alchemy is the cheepest way to reproduce Potions to use during quests or a Battle and the simplest way to learn Alchemy is to eat ingredients to find out the effects good or bad. For more instruction on Alchemy see page 40 of the Installation Guide