I use the th8a from thrustmaster (with the sequential knob add-on), plugged into the t300 base. It's mapped to the paddles. So you can use both. I use another th8a as h shifter plugged directly into the PC.
I have got the Fanatec ClubSport Shifter SQ V 1.5 and I'm very happy with it. It is very solid and gives a really good feedback. You also have the option to switch it to a h-pattern mode instantly just by turning two levers on the side of the casing and for both modes you can adjust the resistance of the rod.
Some years ago I had the Thrustmaster TH8A. I had custom made aluminium SQ and short throw H plates for it, which were a lot better than the originals. But the shifter feel was too light for me (others used rubber bands inside to make it feel stiffer). Also changing the plates every time was too much of a hassle after a while. I then switched to the Fanatec CS SQ V1.5 which I (obviously) use(d) for SQ as well as H-pattern shifting (one click does the job). One minor point (could be personal but I know of several other guys who feel the same): you can tune the spring to be harder or softer (by means of an allen key), which is good. But when set for the H it feels too strong for the SQ and vice versa, which is not good. For that reason I bought the Heusinkveld SQ shifter. That is an awesome piece of work, feels excactly how it should and is very moddable (several sticks and knobs are provided). But I - me again - found an issue using it in AMS. It works perfect in all other sims but in AMS it seems to mess up my cockpit view, it disables all buttons on my wheel and when removed I have to reassign pedals, steering wheel and shifter input. I already asked on the Reiza forum (no answer there) and contacted Niels Heusinkveld to find out if there is a solution. One guy reported he also uses the HE SQ shifter with AMS without issues... Despite that, if you are looking for a dedicated SQ (only) shifter: I would recommend the HE SQ: http://www.h-engineering.net/shop/sim-shifter-sequential/ It would be nice if R3E could provide extra controls for gear UP/gear DOWN (like rF2 does) so you can use either paddles and SQ without having to change profile each time. Using JoyToKey is a solution but not seldom messes up other things.
Use a TH8A myself and I'm happy with it, but just to give you some more options I'm going to mention the SHH shifter. http://www.shiftershh.es/en.html Quicker to switch between Sequential and H-pattern then the TH8A, and a bit cheaper depending on where you live.
Well, the guys at Heusinkveld were very helpful with tips. I finally deleted my controller profiles completely and started from scratch, which led to a working shifter in AMS.
I've bought a used Heusinkveld SQ Shifter a few days ago. It is amazing but pure luxury. I would recommend the SHH Shifter from 2 postings above.
Sequencial Gearshifters in metal with the "Real Feeling" at The SimulatorShop www.prosim.no 3 different models, starting at Exportprice 143 EUR ;-) VIDEO (Youtube "prosim.no" >>
I am not sure of your budget, however 100% recommended is the, Quaife / Pro-Sim Lite Sequential shifter. I have owened many shifters and IMO this one is the best feel. Heusinkveld & Derek Speare also make great SQ shifters. Also i added my own Sparco Alloy shift knob as the standard issue plastic knob was a bit "meeh"
I have experience with: -Fanatec Clubsport Shifter SQ -DSD 2015 Pro Shifter -Heusinkveld Sequetial shifter -The Fanatec was my first shifter, but as soon as you want to raise the restistance it gets stuck. The throw is also pretty long. The size of the Fanatec shifter is huge, really huge. -The DSD 2015 Pro Shifter is a great shifter, really short throw and looks awesome on any rig. I really like how you can adjust it having high restistance, so you have to use a lot of force to change gears. -Finally : The HE shifter. Shifting feel is spot on. The shifer is tiny, very tiny. It's about a quarter size of the Fanatec shifter. I really don't like the looks of it at all. I would recommend the HE shifter, with the bendend rod it feels pretty awesome.