In the winter, I hardly ever heat my pool but I use the attached spa multiple times per week. Even in the summertime, the spa gets used more often than the pool.
But I'm the kind of person who only uses the pool for relaxation. I don't swim laps, I don't have kids, I don't invite people over to play pool games, etc. I mostly just float around and look at the sky. So soaking in the spa, for me, is a lot like what I'd be doing in the pool, except warmer.
If you're not certain that you'll use an attached spa, maybe save the money -- or spend it on a bigger pool -- but leave room (and perhaps run 220V wires and/or pour a concrete pad) for a standalone hot tub in case you decide to install one later.
One thing, though: There's not really any extra maintenance required for an attached spa, since spa and pool share the same water (and can share the same pump, filter, and heater). But a standalone spa requires its own separate maintenance: water balancing and sanitizer addition, periodic draining, purging and cleaning, etc.