interesting requests.. i'll try to help a little, my time is limited, hopefully others will chime in...
first kingston and spanish town.. while the later is near a suburb of the former, they are quite different... spanish town is much more residential and not all parts are good... i do hope you know someone there and have someone showing you around... it's not IMO somewhere a single female should head aimlessly... some communities are reputed to be among the most violent on the island... there are many schemes/areas that are perfectly fine and see no issues... a newcommer would not easily be able to determine where to go... there also is very little (no) commercial tourism what-so-ever... there's an old bridge and some interesting history in the small central town area... but not much else unless you're into a bit more obscure history/arts ect (there's a small taino museum nearby for example)
Kingston has many different colours... there are the more touristy things to do in and around the new kingston area... i highly recommend heading up into the hills to newcastle and those areas... phenomenally beautiful up in the blue mountains... other things to see... marley museum, devon house, emancipation park, port royal (esp if going to limes cay (below)), rockford mineral spa, hope zoo/gardens... there's some decent shopping/dining in new kingston and nearby Halfway tree... and more... but there are bad areas too... again having someone who knows the city accompany you is pretty good idea....
lets see...beaches... in kingston - hellshire and limes cay are the two beachy hot spots...
one (hellshire) is a bit west of the city proper, nearby the portmore area, and very much a hang out into the evening hours, low key vibe...
the other (lime cay) is a bit more isolated and requires a water transfer out a small "island" offshore from port royal...
both are hotspots on weekends/holidays, will be very unpopulated on weekdays
Quad is a very north american style club... it has a great following and always gets a pretty good crowd, but if you've been to an "urban" club in NY, DC, Miami ect, you will not feel like it's all that different... there's a newer place Plush: Pure (or is it Pure: Plush lol) anyways, same area... so if one isn't happening you can always jump next door... personally i prefer the outdoor parties more... there's usually something keeping most weekends (some are bigger/better than others), you'll hear/see plenty of advertisements for different things... some are in good areas, some are not, you'd need someone with knowledge of the city/scene to tell you which ones are "good"... i'm a fan of the more upscale ones (hienkein parties are always nice, rhythm and booze, twisted spirtz, french connection, ect ect ect, depending on the time of year you go things may be busier)
as for comparing brazil and jamaica, well i've never been to brazil, but i think like anywhere it depends on where you go... it's a big country and I'm sure it has it's bad parts which you or may not have experienced... I know some parts of brazil are very visitor-friendly and not considered unsafe at all for the average person with reasonable intellegence

.. the same goes jamaica.. with some common sense and even more helpful, someone familiar with the areas you are going, you can explore safely...
as for hospitality of the locals and such... from my experience this is best found in two places, first being the touristy areas your agent has pushed on you (negril, mobay ocho rios) typically at a price/agenda (they want tips or some sort of benefit out of it, obviously not all, but the closer you are to tourism, the more this is a rule and not an exception)... then you find it in the smaller rural communities where people look out for one another... they treat visitors as guests... (again, some may want something but it becomes more an exception and less a rule)... in kingston you'll find that like in any big city, most are just going about their lives and trying to make it thru the day... you'll be harder pressed to find the bend over backwards hospitality... it exists, but is harder to find...
so if you're looking for that, do try to make it into the more off the beaten path type places if you can... i.e. St Thomas i have found to have some of the nicest people on the island... it's about as far removed from both tourism and the city bustle as you can get
okay.. that was a ramble, hope it helped some