So, comparing some math here
1) AT-ST main gun should be 1.5 + 1 + 0.5 = 3 Hits unmodded. If you add the light blaster (1 of each die), you get 4.5 hits unmodded.
1 squad of Bikes should be 1.75 + 1.25 + .75 = 3.75 Hits unmodded. Thus, 2 squads taking shots would be an insane 7.5 Hits unmodded!
2) The AT-ST will have impact 3, then impact 4 if you add the light blaster.
1 Squad of Bikes will total impact 2, while 2 would thus total impact 4.
3) AT-ST with light blaster costs 215 squad points, 2 bike squads will be about 180 to 210 squad points.
4) Both AT-ST and Bikes have a white defense die (33.3% Block), though the AT ST has Armor and the Bikes Cover 1.
Soooo, with that in mind, some thoughts:
> Once a bike squad takes 3 damage, its unmodded offense drops to 1.875 Impact 1 (for one bike). An AT ST has to take 9 damage before it has a small chance of losing a weapon, so likely its offense will stay full power until 11 damage.
> AT-ST has strong melee, the bike has no melee.
> AT-ST can displace at will and provides cover to troops. The Bikes can only displace on a compulsory move, and do not provide cover.
> The AT-ST can stand still and own a section of the map. It can aim and shoot, or move and shoot. It can hit one squad multiple times. The compulsory move makes this difficult for the bike to do. You can move, take a shot, but next turn you will have to move again. You can 90 degree turn for compulsory as the bike, and then use a speed 1 to 90 degree again, but that will depend on terrain, etc.
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So, it seems taking two bike squads vs taking an AT-ST comes down to play style. I play Rebels in X-Wing, so I personally prefer slow-roll tanky units, but the Bikes definitely have a stronger alpha strike (assuming you can keep all 4 alive to shoot at the same target). The AT-ST will provide more consistent damage throughout the game, as it is likely to live the whole time.