A Slim Jim and a J pole have essentially the same performance. This shouldn’t be a surprise, they are both half wave antennas.
Working with twin-lead, I recommend the Ed Fong dual band J pole. That gives you 2 m and 70 cm in the same antenna. You can buy pre-made antennas from Ed or you can make your own. Scroll to the bottom of this page and he’s added links to the QST articles describing the antennas.
wunder