The germans call it usually Bankfurt or Mainhattan (Main is the river going through).
There are more beergarden-type places (though locally they are more fond of apfelwein=a kind of cider) in the south of the river part, near Süd BHF (Sachsenhausen). (the above was just secs before I wrote this down).
Quoted Marburg is well over 1 hr away by train.
Tip: if you decide to stay for 2 days, google for Rhein-Main Card, which is a 2-day ride-at-will tourist ticket for all of the quite large VRM=Local tariff Union verbund, cost 22€ and can be ordered online or bought at some tourist offices-NOT from DB! As with all german such tickets, you can use it at will, without any 9.00 rule or so, on LOCAL (=non IC/ICE) trains from DB and the various other contracted operators. Its in fact cheaper as the bought from machines all-area dayticket!
Incidentally i just checked HTL-prices in BRU for some weekends and they are certainly higher as FRA for lesser quality, BUT; as again anywhere in germany when there is a big fair (messe), at those dates prices go up a lot.
In srping I stayed for 2 nights weekendrate at HotelBB, just steps from Main HBF, for just 99€, BF is extra. This HTL chain is like travelodge/premierlodge, will often not appear in sites as booking. Rooms always have AC and a big TV. Also check hrs.de for HTL-bookings in DE, as its more locally covered as booking or expedia.