I'm currently using a two-handed weapon and wanted to try out high DPS one-handed weapons in the store. I can't tell if I can use the large white numbers to off-handedly compare DPS (pun unintended?). I.e., if I am ignoring bonuses from attributes (class DEX/INT/STR, attack speed, extra crit damage), will the sum of two one-handed weapons' big numbers exceeding a two-handed weapon's big number indicate an increase in DPS? Or is it largely useless/futile to compare weapons this way?
The big white number is the weapons DPS. For the most part, you won't go far wrong by always using the weapon for which this is higher, since it takes into account the weapons base damage, any bonus elemental damage, its attack speed, and any attack speed bonuses on the weapon. When dual wielding, you should average the DPS of both your weapons, then multiply by 1.15 to get a number for comparison to your 2H.
All that said, as you increase in level, the potency of having a weapon with points in your primary stat increases dramatically, to the point where a weapon with slightly lower DPS may still be worth using. The best guide for whether a weapon is an upgrade is not the large DPS number, but the Damage category in the Stat Changes If Equipped summary at the bottom of the tooltip. For more on how that's calculated, see here: