Of the various boxes referred to in the links above, I would say that the
Alpha Strike Boxed Set makes a solid case for being a good first jump-on point - both in terms of how many 'Mech minis you get for the price, but in how it supports both
Alpha Strike play (using the rules and Unit Cards in the box) and the opportunity to later branch over to
Total Warfare scale play (not least since CGL have conveniently compiled
a PDF with Record Sheets to match the aforementioned Unit Cards).
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Actually, there are a few differences between
TW and
AS which might make some 'Mechs more, or less, useful in certain roles than others, depending on which game system is being used.
For example: both games have the concept of a to-hit modifier based on the movement of the target unit. However, in
TW, this Target Movement Modifier depends on how fast the unit actually moves, increasing at certain move brackets; in
AS, there is but a single TMM modifier per unit, which is the same even if the unit moves the bare minimum amount needed to trigger it.
How big a deal is this? I defer to the experts, in terms of actually playing both games on the tabletop. But, again, the variety of units in the
AS Boxed Set might help you gauge what a helpful variety of different Inner Sphere and Clan designs each have to offer in this regard.