Either one will give you a good cardio workout depending upon YOU. Rollers are better for me, because I have to worry about balance and keeping focused on riding. On the trainer it is very, very boring for me.
Inexpensive is a relative term in the bicycle community. I used craigslist and got my rollers for $50.00 and have used them for two winters and planning on a third. I have about 3000 miles on the rollers since I have owned them.
Have fun with whichever you chose.
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2007 Jamis Ventura Comp
2006 Jamis Explorer 2.0
2000 Specialized Hardrock (bought used)
Swim, Bike, Run and sounds like fun