Just my 2 cents- You can see by my signature I've had my share of Spyders. With regards to the 998 , it is actually the second motor used in the Spyder , first being the 990. The 2008-2009 spyder shared its powertrain with the Aprilia RSV motorcycle and the motor was first introduced in 1997, yes 1997. It has a long , race proven pedigree. . The original spyder made 106 horsepower and 77 pound-feet of torque and the horsepower was well into the upper bands of the rpm range. . The V-twin did lose one of the Aprilia’s six forward gear ratios to enable reverse. So when the RT was introduced in 2010, BRP tasked Rotax with reinventing the 990 and it was rebranded as the 998 , while sharing 70% of the same components with the 990, was far smother and had slightly less horsepower (100 hp vs 106) but 77 ft pounds of torque that came in at a much lower RPM vs the 990 . The current 1330 triple was purpose designed based on a variant of the triple rotax used in the entry level seadoo and is 115 hp and 96 ft pounds of torque but weighs considerably more than the 998.
Currently in our stable is a 2013 ST LIMITED , a 2103 RT LIMITED and a 2008 GS. We sold our 2020 RT Limited (my second 1330 engine based RT) early 2021 . As odd as I may be , I prefer the V-twin. I would put the 13 RT limited up against my 2020 for smoothness and quietness ( and drag race, it would be very very close) anyday. Yes I got 38 mpg with my 20 and only getting 34-35 with my 13 but other than that I can't find a fault. The 2020 RT was my wifes and she rode her 13 ST Limited almost 3 times as much , she prefers a little more open air ride. As for parts and worries of parts issues etc, that 998 motor is beyond bulletproof , its proven itself over and over and is literally Goldwing reliable. Parts will be around a very long time. Not sure if you are looking at it from a power/performance issue , but the ST will clean the RT's clock in straight line acceleration, only 9 hp less and very much lighter. (970 pounds versus 760)
Buy the ST , enjoy the heck out of it and go ryde some more!