are you currently using umobile on your phone or other device? LTE is a big jump from HSPA+, but if your area is not covered, then you'll not be fully utilising the LTE mifi.

i suggest for you to try a prepaid sim on an LTE enabled phone, and check whether the places that you'll be using it has Umobile LTE coverage, and check the average speed.

frankly speaking, LTE coverage wise, Maxis is leading the game. so you might want to try a prepaid maxis on your lte device too before deciding on which carrier to use.
on the question whether is LTE worth it? yes it is. faster than unifi 20mbps.

the max i got with DC-HSPA+ was 20.++

yes, 99 / giant / 7-11 / cosway ..

u just want test at usb modem then no point u subscribe umi plan as it comes with call and sms bundle with data... if jsut want to test 1 day then got for daily rm5 will do , can immediately finish up the amount inside the sim also. else u need top up another 10 then only can subscribe the mb15.

different between HSPA and HSDPA

HSPA = higher download speed (HSDPA ) + higher upload speed ( HSUPA )
HS D+U PA

HSDPA = higher download speed + slower  upload speed of 384kbps WCDMA 3G
for weaker signal wise, you can either buy active USB extension cable to put your usb modem at near window,..

or

buy a 3G/4G router, plug your usb modem in and put them near window

or Invest in Antenna!!!!