I've come back to Sonar after some time away (8 years!), and am a guitar player. 
 
I'm a home/hobbiest, and for my needs an amp sim has a lot of advantages (convenience?) over micing an amp in my apartment. 
 
Don't get me wrong, I prefer amps, but for recording a sim/modeler works better for the flow. 
 
Back in the day, I was a fan of the Line6 POD.  It was well controlled in Sonar, was flexible and worked well in the mix.  It also seemed to have a hand up on the plug ins at the time. 
 
But now it's 2016, and there are better modelers out there (Kempler, Axe-FX, Helix), but those are currently out of budget. 
 
I have a new PC, have you found any sims that are better than the older hardware? 
 
There seems to be 2 schools of thought with sims, TH3 comes with everything, and products like Amplitube that make you purchase every feature.  Some amps might be better on this or that product. 
 
My needs are simple - I need a good NMV Fender and Marshall.  A nice selection of boosts would be nice
 
What are you using? 

Atomic Amplifire is very good for $599.
For sims I prefer S-Gear 2.52, and Amplitube 4...I also have Revalver 4.

^^^What he said...

If you've got SPlat I'd give the included TH3 a try first before spending any additional $$.   Oh, and the CA-X amp sims (under FX chains).  Between those you have some good options.   But if you then find that they're still not giving you quite what you're looking for then you can demo some 3rd party plugins (like S-GEAR). 
 

I've been using NI Guitar Rig for years, but recently picked up Amplitube 4.

GR is great for complicated rigs, they have lots of decent amps, effects, and tools.

For a plug&splay setup, I'm loving Amplitube 4. A4 has sims licensed by the amp manufacturers, and they're more realistic than GR, but I haven't seen the flexibility in A4 that some may want.
A4 also allows you to buy individual amp sims - I picked up the Fender Twin Reverb sim recently, and it's fantastic. The Marshall sims in A4 are good too. Plus, they have a "try" option where you can download a specific amp sim for a few days to decide whether it's what you want!

GuitarRig is my first choice.  The improved sounds of amplitube didn't offset the difference in flexibility of GuitarRig.

Though currently out of budget, Helix is a great solution (far beyond the POD series).
Has greatly simplified both my live rig and guitar/bass recording...
 

• UI is amazingly simple (you won't ever need the manual)
• New modified Marshall JCM800 is very good
• There's a good selection of drives
• You can load custom cab IRs
• Lots of I/O options to work with the rest of your gear/studio
• Flexible signal routing
• "Snap-shots" (similar to Scenes in Fractal Axe-FX and AX8)

 
I've had the Kemper, Axe-FX XL+, AX8, and Helix.  
Great sounds can be had with any of them.
All are IMO (currently) a bit better than their software plugin counterparts.
FWIW, I consider Helix one of the best musical purchases I've ever made.
If funds are tight, Zzounds or AMS offer 12 months SAC.
 
There's talk that Cliff (owner/developer from Fractal) might release a plugin version of the Axe-FX sims.
He's already released the reverb as a VST Plugin.
 
 

Over time I've collected a few sims in bundles or during sales.  GR5, AT4, Waves GTR, TH2/3.
 
I've used GR5 a lot.  Then I went to GTR for a while and now AmpliTube with the Mesa bundle.
 
I also have a Laney IRT Studio amp cabled up for direct recording.  I really like this amp feeding cab/mic sims in one of the mentioned sims.  Also in hardware, the BOSS GT-100/001 sound is very good.  I have the -001 (desktop) and like it a lot as well.
 
IMO, they all have pros/cons.  GR5 is probably the most complete, but is getting dated.  GTR was a total surprise to me when I bought Waves Gold.  IMO, GTR is a real gem and is also quite complete.  I can't get past the clumsy interface of TH3, although what little I've tried has sounded really good to me.  AmpliTube 4 Mesas are in my current project.
 
I think it would be difficult to make a really bad choice ;)  And don't rule out configs like going direct with GR5 in front for effects followed by another for the amp.
 
Speaking of which ... going all software lets you record direct and then waste hours and hours looping and tweaking your sim to get things just right -
 
Watch the Deals forum ...

My Hughes & Kettner Grandmeister has XLR out with Redbox built in
for direct recording...that's a pretty sweet feature....

cclarry
My Hughes & Kettner Grandmeister has XLR out with Redbox built in
for direct recording...that's a pretty sweet feature....

Those H&Ks have been tempting me forever.  If the 18 had reverb I would prolly own one.
 
My Laney has built-in direct support as well.  I have a Red Box 5 I use with my Bugera V22 from time to time.  The thing I like about the Laney IRT is that the direct out has switchable emulation, which allows me to feed ITB cab/mic sims.
 
I use the RB5 with my HT5RH too.  It has direct out, but it's basically just the pre routed thru emulation.  Sounds good though.
 
So many options these days.