Grey Owlite Shield outclasses Heater Shield majorly, both for defense and the fact that Fire + Shock resistance is much more useful than Freeze resistance.

I can relate, I actually had a Mighty Defender as my first 4* shield, and I wanted to upgrade it to Heater Shield, but I knew I would probably want a shield that was better in the long-run. So I started my shields over and actually made Grey Owlite instead. In fact, I only upgraded my Mighty Defender just recently, only because I had some spare Eternals to waste. It's really not worth it over Grey Owlite from my experiences so far.

go for aegis if you're really cheap and don't care about your shield.

If you want a proper shield...

For a true all-rounder, go with volcanic plate or omega shell. they both have very good normal defence, and omega shell has about 20% extra "shield health" compared to other shields. These are the only two shields in the game that can withstand a hit from Lord Vanaduke's mace on Elite.

Damage type resistance is not that important, it has been tested and proven that decent defence against a damage type only changes it by about 2%.
Therefore, status resistance takes a large priority over damage type resistance, as astatus resistance makes the shield completely immune to said status.

For example, in the firestorm citadel, where you will be spending most of your time, there is lots of fire.
Volcanic plate shield and Owlite shields are immune to fire, meaning that they will not break at all from the fire zombie breath/vana's fire attack, will take less damage from any attack that would normally inflict fire.

However, you can go the other way about it, and go with the whole "glass cannon" idea. In which case you will want either a swiftstrike buckler (For universal attack speed increase) or a Snarbolax-shield-that-I-can't-spell-the-name-of (Sword only damage increase). I have used the swiftstrike, and can say that unless you're pretty confident on being able to dodge, you should probably stick to a "proper" shield.

I hope that this hasn't become a TL:DR, and that it has helped you make a more informed descision on what shield you would like.

(Morale of the story: Get volcanic plate, owlite or swiftstrike)

The snarby sheild is a barbarous thorn sheild.

Well technically you buy the bristling buckler off brinks for snarby boss tokens.

NOT TRUE! My level 5 aegies once took a blow and didnt even go down to yellow! (Mabye his hits are bugged or something) But every shield ive had can stand a mace blow