How can I differentiate Hell Hounds and Spiders in my turn-based strategy game?

Okay, so the Barracks is the first combat production structure, and with some unlocks, it produces Commandos (Basic Infantry), Hell Hounds (Ground-only anti-small infantry), and Mages (Expensive, but powerful spell caster).

The problem I'm having is Hell Hounds fill a similar niche to the Spider, which is the new name i have for the Factory unit which is anti-small infantry too.

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Spider

Gold Cost: 75

Oil Cost: 25

Build Time: 3

Supply Cost: 2

HP: 100

Armor: 1

Attack (Ground) D10, 2D10 vs Small

Move: 3

Mechanical Medium Vehicle

Cargo Size: 2

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Hell Hound

Gold Cost: 50

Oil Cost: 50

Build Time: 2

Supply Cost: 1

HP: 60

Armor: 1

Weapon (Ground Only): D10, 2D10 vs Small

Move: 2

Biological Small Infantry

Cargo Space: 1

Heal

Hell Hounds naturally heal by +5 HP per round.

===

Gold is usually the limiting resource, because everything costs Gold, but not everything costs Oil.

Cargo space determines how many units can stand on a given space on the map. The map grid spaces each have a capacity of 12 so a unit with a cargo space requirement 1 can stack 12 units on the grid space, while a vehicle with cargo space requirement 2 can only stack 6 units in a grid space.

Neither of these combat units can attack Air, so they are vulnerable late game to air assaults. This means you can't afford to over-invest in ground-only specialist units forever, or you'll eventually lose.

What might i do to make these units more unique from one another?

Updates
14 d
By the way, a Barracks costs 150 gold and 50 Oil to produce.
A factory costs 250 gold and 150 oil to produce, so more up-front costs, which makes it slightly harder to mass vehicles than infantry. Star Base also costs 250 gold and 150 oil, so typically to go air units you'd need to be ahead by an entire base.
How can I differentiate Hell Hounds and Spiders in my turn-based strategy game?
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