The VTF 15H Mk2 is a bigger box with lower port tuning giving it deeper extension for lower frequency.
Low frequencies are more affected by room volume whereas your regular speakers are more dependent on distance to your LP. That's why room size comes up so much in these conversations.
If you wanted to "play it safe" and you are not needing to hear that low c on a pipe organ, the VTF3 is a really good sub. A lot of people really like them which is why they are so often recommended.
The Monoliths were reviewed here and are proving themselves to be quite worthy, as well.
You won't necessarily go wrong with either decision.
At the end of the day... why don't you just set the money aside for when you find the right spot? You might find you don't need that much sub... Though everybody needs 2 or 3 of that much sub!
But seriously, there is so much more at play in the way a subwoofer performs than just picking one with good specs. Conversely, you might find the right subwoofer only to realize that you need two to overcome issues in the room.
