Gotta be honest, I don't think the SRX300 are anywhere near as good as the SR300, in terms of feel, sound, and build quality.
If you want a cheap, versatile, quality starter bass, I would look for a used Yamaha RBX270. Awesome basses for about £100 (give or take) used, and easy to modify. I've got one that I use as a backup/practice bass. I upgraded the pickups, wiring and bridge, and it's killer! I would gladly play it on any gig.