Ultimately based on Alleycat's research (check link below), I'd say hoplites.

With almost everything being relatively equal, (According to the brilliant Alleycat see thread Hit Points) I think ultimately the deal breaker is the cost of upkeep and how much more bang for your buck you get when selecting hops over steams.

For example with normal numbers.... ("normal" being roughly a 80k point player)

Domestic Defense (Your OWN towns no garrison\attacking cost doubles)

70 SGs on a front line would cost 840

210 Phalanx on a front line would cost 630

More realisticly let's assume the average player who has the ability to create steam giants has roughly 5 towns and a capital. For the purpose of this number crunch let's assume each town has just enough units to fill a metropolis defense (7 front line, etc etc.)

70 SGs on a front line in all cities would cost 5040/hour

210 Phalanx on a front line in all cities would cost 3780/hour

Now the difference between 5040 and 3780 is 1260. Savings of 30240 gold per day.

With 1260 more gold to toss around you are able to support 420 more hoplites for the same cost as having just the 420 SGs.

That's a total of 1680 Hoplites for the same price (in gold) as 420 SGs.

Combined with the fact that Hoplites can benefit from doctors, this clearly makes Hoplites superior. You have morale benefits to consider from having a greater amount of units, the fact that because of the smaller size of hoplites, they don't get wiped out by mortars as quickly. With that in mind also you lose fewer resources when the mortars one shot your precious SGs.

In case you're not convinced here's what one of those oldschool maxed out Alpha players number's would be like if they chose one over the other. Let's assume the tip of the top dog has 11 towns and the 1 capital (maximum currently possible) And they have enough for the front line and then two more full replacements for them (420 more hops, 140 more SGs)

2520 Steam Giants costing 30240 gold/hour

7560 Hoplites costing 22680 gold/hour

Do the same math as before and you've got the possibility of 2520 MORE hoplites for the same cost as just 2520 SGs.

10080 Hoplites @ 30240\hour

2520 SGs @ 30240\hour

And just think. These numbers apply when the said units are posted in your own cities. They DOUBLE when on the move\occupying\in garrison\attacking.

***Please note that all numbers posted here are made without any consideration for the cost-reducing technologies one will have had to research in order to gain access to Steam Giants. Also not that no Military Future Techs are considered either.**