Hello, before I start I would like to say that I admit that I'm just an average player at best and I am not a high level player or anything so please correct me if I am wrong. I would like to improve my play by learning more about the game.
I have played 47 games with Taric to date with 28 wins and 19 losses and I think I have tried about every stock build and also experimented with a few of my own builds including the Aura Itch build, the dps build, hybrid, and tank.
I found most success in the Aura build.
But recently I picked up Sion and started to play him just for fun and I feel as if Sion completely outclasses Taric in almost every way possible. Please correct me if I'm wrong but here are my observations.
Taric's Stats:
X Health 468 (+90 / per level)
X Mana 255 (+56 / per level)
X Move Speed 315
X Armor 16.5 (+3.2 / per level)
X Spell Block 30 (+0 / per level)
X Critical Strike 2 (+0.25 / per level)
X Health Regen 1.42 (+0.1 / per level)
X Mana Regen 0.82 (+0.08 / per level)
Sion's Stats:
X Health 473 (+104 / per level)
X Mana 175 (+25 / per level)
X Move Speed 310
X Armor 17.75 (+3.25 / per level)
X Spell Block 30 (+0 / per level)
X Critical Strike 2.2 (+0.3 / per level)
X Health Regen 1.62 (+0.2 / per level)
X Mana Regen 0.76 (+0.04 / per level)
My observation is that from pure stats alone, I already see that Mana and a negligible amount of Movement Speed is the only thing Taric has going for him
In addition to Sion's superior stat growth, Sion has his ability Enrage that gives him 3 permanent health for each kill. If I average about 80-100 minion kills per game, that equals about 240 to 300 additional health for Sion.
Abilities:
Cryptic Gaze VS Dazzle
Cryptic Gaze does 70/125/180/240/300 damage and stuns for two seconds.
Dazzle does 80/160/240/320/400 damage and stuns for two seconds max.
My opinion is that I am usually at max range when I stun anyone so damage Dazzle basically does no damage and stuns for the same duration as Sion's stun.
Sure some would argue that Cryptic Gaze requires a larger fraction of his mana pool to cast, but atleast it does some damage for that cost.
I also believe that Cryptic Gaze has a higher travel time which increases its usability as a chasing move?
Although Cryptic gaze has 100 less range than Dazzle does but with Ghost/flash it wouldn't matter all that much.
Death's Caress vs Shatter
Death's Caress protects him for 120/170/220/270/320 damage and does 120/170/220/270/320 damage.
Shatter gives a 30 armor aura and does 100/150/200/250/300 damage and reduces armor by 30.
Honesty, I think that Death's Caress is better than Shatter because it protects him for flat 300 damage instead of just giving him resistance to physical damage only. It's more of a general defensive ability if I want to run up and start initiating.
I guess you wouldn't get the damage from the shield if it was prematurely popped. Maybe Shatter is better? Maybe not?
Enrage VS Imbue
Enrage gives Sion 1/1.5/2/2.5/3 per kill and gives him 25/35/45/55/65 extra damage per hit.
Imbue heals for 70/120/170/220/270 health.
Okay, these two abilities aren't very comparable but I believe that if I was to get any usability out of Taric, I HAVE to max out Shatter and Dazzle first for the armor/damage and the dazzle cooldown reduction. This means that I only have 1 level of Imbue for most of the game making the heal useless. Maybe other people found a better use for this heal. Correct me if I'm wrong.
However, Sion's Enrage gives me about 240-300 more hp per game which is pretty cool.
Then, let's add the ults into the equation:
Cannibalism VS Radiance
Cannibalism gives Sion 50/75/100% lifesteal and 50% attack speed for 20 seconds, additionally Sion's attacks heal surrounding allies for 25/37.5/50% of damage dealt.
Radiance heals everybody for 30/40/50 health a second and increases his attack damage by 30/60/90 and 15/30/45 for teammates.
Firstly, I'd like to say that the healing from Radiance is negligible at best. I found it completely worthless. It heals at most 2 bars of health while if you were to manage your mana bar to have enough mana to cast dazzles and shatters. And it leaves you with no more mana.
Radiance gives Taric a damage boost whenever he has Radiance on, but Sion gets a little less damage boost for absolutely no mana cost with Enrage.
Sure Radiance gives his teammates a boost, but it drains Taric's mana bar way too quickly.
Plus, when Sion has Cannibalism active, he is basically indestructible. And lasts 20 seconds for just 100 mana while trying to keep Radiance up for 20 seconds would cost 2100 mana while leaving you with no mana to cast Dazzle and Shatter.
Radiance, as great as it is as a pocket Rally, just makes no sense. If the cooldown for Radiance was reduced to about 1/2-3/4 of its normal cooldown, maybe it would be easier to manage his mana pool by toggling it on and off during battles.
In addition to Sion's lifesteal boost, he gets a rediculous attack speed boost to lifesteal even more! It's wonderful coming out of a teamfight with more than half my hp left and enough of my ult duration left to push a tower quickly.
These are the reason why I think Sion completely outclasses Taric.
Taric feels like he's pulling less for the team than Sion is.
However, don't get me wrong. I really love Taric. I want to believe that he is better than I think he is because I would rather play Taric instead of Sion and somehow make him even better than Sion. But I just see no solution.
Please offer me some advice on what I can do to improve my play as Taric.
Thank you for taking your time to read this 
I will appreciate your constructive criticism.