I see room for both.
The standard engine 8/12 Locust, in, for example, the LCT-1V or LCT-1E layouts, are just about the most durable 20-ton bug 'Mechs possible, and move fast enough to generate to make them very tough to hit.
With that said, though, faster is sometimes better, and the super fast Locusts bring other options to the table, in that they can often get outside practical weapons range entirely when they lose initiative.