Definitely buy the PME tips. I would search online as several sites may offer the tips cheaper. You will see a difference when using PME tips vs Wilton tips. The Wilton tips cause the RI to curl and you will notice a seam. The PME tips have no curl and show no imperfections in your string work. If you buy the Wilton tips make sure you clean them and dry them right away as they do discolor. The PME tips do not discolor and look flawless as the day you bought them.
Hope this helps!